SQL - Quick Guide

SQL - Overview

SQL is a language to operate databases; it includes database creation, deletion, fetching rows, modifying rows, etc. SQL is anANSI(American National Standards Institute) standard language, but there are many different versions of the SQL language.

What is SQL?

SQL is Structured Query Language, which is a computer language for storing, manipulating and retrieving data stored in a relational database.

SQL is the standard language for Relational Database System. All the Relational Database Management Systems (RDMS) like MySQL, MS Access, Oracle, Sybase, Informix, Postgres and SQL Server use SQL as their standard database language.

Also, they are using different dialects, such as −

MS SQL Server using T-SQL,
Oracle using PL/SQL,
MS Access version of SQL is called JET SQL (native format) etc.

Why SQL?

SQL is widely popular because it offers the following advantages −

Allows users to access data in the relational database management systems.
Allows users to describe the data.
Allows users to define the data in a database and manipulate that data.
Allows to embed within other languages using SQL modules, libraries & pre-compilers.
Allows users to create and drop databases and tables.
Allows users to create view, stored procedure, functions in a database.
Allows users to set permissions on tables, procedures and views.

A Brief History of SQL

1970− Dr. Edgar F. “Ted” Codd of IBM is known as the father of relational databases. He described a relational model for databases.
1974− Structured Query Language appeared.
1978− IBM worked to develop Codd’s ideas and released a product named System/R.
1986− IBM developed the first prototype of relational database and standardized by ANSI. The first relational database was released by Relational Software which later came to be known as Oracle.

SQL Process

When you are executing an SQL command for any RDBMS, the system determines the best way to carry out your request and SQL engine figures out how to interpret the task.

There are various components included in this process.

These components are −

Query Dispatcher
Optimization Engines
Classic Query Engine
SQL Query Engine, etc.

A classic query engine handles all the non-SQL queries, but a SQL query engine won’t handle logical files.

Following is a simple diagram showing the SQL Architecture −

SQL Commands

The standard SQL commands to interact with relational databases are CREATE, SELECT, INSERT, UPDATE, DELETE and DROP. These commands can be classified into the following groups based on their nature −

DDL - Data Definition Language

Sr.No.Command & Description1CREATECreates a new table, a view of a table, or other object in the database.2ALTERModifies an existing database object, such as a table.3DROPDeletes an entire table, a view of a table or other objects in the database.

DML - Data Manipulation Language

Sr.No.Command & Description1SELECTRetrieves certain records from one or more tables.2INSERTCreates a record.3UPDATEModifies records.4DELETEDeletes records.

DCL - Data Control Language

Sr.No.Command & Description1GRANTGives a privilege to user.2REVOKETakes back privileges granted from user.

SQL - RDBMS Concepts

What is RDBMS?

RDBMS stands forRelationalDatabaseManagementSystem. RDBMS is the basis for SQL, and for all modern database systems like MS SQL Server, IBM DB2, Oracle, MySQL, and Microsoft Access.

A Relational database management system (RDBMS) is a database management system (DBMS) that is based on the relational model as introduced by E. F. Codd.

What is a table?

The data in an RDBMS is stored in database objects which are called astables. This table is basically a collection of related data entries and it consists of numerous columns and rows.

Remember, a table is the most common and simplest form of data storage in a relational database. The following program is an example of a CUSTOMERS table −

SQL CODE

+----+----------+-----+-----------+----------+
| ID | NAME     | AGE | ADDRESS   | SALARY   |
+----+----------+-----+-----------+----------+
|  1 | Ramesh   |  32 | Ahmedabad |  2000.00 |
|  2 | Khilan   |  25 | Delhi     |  1500.00 |
|  3 | kaushik  |  23 | Kota      |  2000.00 |
|  4 | Chaitali |  25 | Mumbai    |  6500.00 |
|  5 | Hardik   |  27 | Bhopal    |  8500.00 |
|  6 | Komal    |  22 | MP        |  4500.00 |
|  7 | Muffy    |  24 | Indore    | 10000.00 |
+----+----------+-----+-----------+----------+

What is a field?

Every table is broken up into smaller entities called fields. The fields in the CUSTOMERS table consist of ID, NAME, AGE, ADDRESS and SALARY.

A field is a column in a table that is designed to maintain specific information about every record in the table.

What is a Record or a Row?

A record is also called as a row of data is each individual entry that exists in a table. For example, there are 7 records in the above CUSTOMERS table. Following is a single row of data or record in the CUSTOMERS table −

SQL CODE

+----+----------+-----+-----------+----------+
|  1 | Ramesh   |  32 | Ahmedabad |  2000.00 |
+----+----------+-----+-----------+----------+

A record is a horizontal entity in a table.

What is a column?

A column is a vertical entity in a table that contains all information associated with a specific field in a table.

For example, a column in the CUSTOMERS table is ADDRESS, which represents location description and would be as shown below −

SQL CODE

+-----------+
| ADDRESS   |
+-----------+
| Ahmedabad |
| Delhi     |
| Kota      |
| Mumbai    |
| Bhopal    |
| MP        |
| Indore    |
+----+------+

What is a NULL value?

A NULL value in a table is a value in a field that appears to be blank, which means a field with a NULL value is a field with no value.

It is very important to understand that a NULL value is different than a zero value or a field that contains spaces. A field with a NULL value is the one that has been left blank during a record creation.

SQL Constraints

Constraints are the rules enforced on data columns on a table. These are used to limit the type of data that can go into a table. This ensures the accuracy and reliability of the data in the database.

Constraints can either be column level or table level. Column level constraints are applied only to one column whereas, table level constraints are applied to the entire table.

Following are some of the most commonly used constraints available in SQL −

NOT NULL Constraint− Ensures that a column cannot have a NULL value.
UNIQUE Constraint− Ensures that all the values in a column are different.
PRIMARY Key− Uniquely identifies each row/record in a database table.
FOREIGN Key− Uniquely identifies a row/record in any another database table.
INDEX− Used to create and retrieve data from the database very quickly.

Data Integrity

The following categories of data integrity exist with each RDBMS −

Entity Integrity −There are no duplicate rows in a table.
Domain Integrity −Enforces valid entries for a given column by restricting the type, the format, or the range of values.
Referential integrity −Rows cannot be deleted, which are used by other records.
User-Defined Integrity −Enforces some specific business rules that do not fall into entity, domain or referential integrity.

Database Normalization

Database normalization is the process of efficiently organizing data in a database. There are two reasons of this normalization process −

Eliminating redundant data, for example, storing the same data in more than one table.
Ensuring data dependencies make sense.

Both these reasons are worthy goals as they reduce the amount of space a database consumes and ensures that data is logically stored. Normalization consists of a series of guidelines that help guide you in creating a good database structure.

Normalization guidelines are divided into normal forms; think of a form as the format or the way a database structure is laid out. The aim of normal forms is to organize the database structure, so that it complies with the rules of first normal form, then second normal form and finally the third normal form.

It is your choice to take it further and go to the fourth normal form, fifth normal form and so on, but in general, the third normal form is more than enough.

First Normal Form (1NF)
Second Normal Form (2NF)
Third Normal Form (3NF)

SQL - RDBMS Databases

There are many popular RDBMS available to work with. This tutorial gives a brief overview of some of the most popular RDBMSâs. This would help you to compare their basic features.

MySQL

MySQL is an open source SQL database, which is developed by a Swedish company â MySQL AB. MySQL is pronounced as “my ess-que-ell,” in contrast with SQL, pronounced “sequel.”

MySQL is supporting many different platforms including Microsoft Windows, the major Linux distributions, UNIX, and Mac OS X.

MySQL has free and paid versions, depending on its usage (non-commercial/commercial) and features. MySQL comes with a very fast, multi-threaded, multi-user and robust SQL database server.

History

Development of MySQL by Michael Widenius & David Axmark beginning in 1994.
First internal release on 23rdMay 1995.
Windows Version was released on the 8thJanuary 1998 for Windows 95 and NT.
Version 3.23: beta from June 2000, production release January 2001.
Version 4.0: beta from August 2002, production release March 2003 (unions).
Version 4.1: beta from June 2004, production release October 2004.
Version 5.0: beta from March 2005, production release October 2005.
Sun Microsystems acquired MySQL AB on the 26thFebruary 2008.
Version 5.1: production release 27thNovember 2008.

Features

High Performance.
High Availability.
Scalability and Flexibility Run anything.
Robust Transactional Support.
Web and Data Warehouse Strengths.
Strong Data Protection.
Comprehensive Application Development.
Management Ease.
Open Source Freedom and 24 x 7 Support.
Lowest Total Cost of Ownership.

MS SQL Server

MS SQL Server is a Relational Database Management System developed by Microsoft Inc. Its primary query languages are −

T-SQL
ANSI SQL

History

1987 - Sybase releases SQL Server for UNIX.
1988 - Microsoft, Sybase, and Aston-Tate port SQL Server to OS/2.
1989 - Microsoft, Sybase, and Aston-Tate release SQL Server 1.0 for OS/2.
1990 - SQL Server 1.1 is released with support for Windows 3.0 clients.
Aston - Tate drops out of SQL Server development.
2000 - Microsoft releases SQL Server 2000.
2001 - Microsoft releases XML for SQL Server Web Release 1 (download).
2002 - Microsoft releases SQLXML 2.0 (renamed from XML for SQL Server).
2002 - Microsoft releases SQLXML 3.0.
2005 - Microsoft releases SQL Server 2005 on November 7th, 2005.

Features

High Performance
High Availability
Database mirroring
Database snapshots
CLR integration
Service Broker
DDL triggers
Ranking functions
Row version-based isolation levels
XML integration
TRY…CATCH
Database Mail

ORACLE

It is a very large multi-user based database management system. Oracle is a relational database management system developed by ‘Oracle Corporation’.

Oracle works to efficiently manage its resources, a database of information among the multiple clients requesting and sending data in the network.

It is an excellent database server choice for client/server computing. Oracle supports all major operating systems for both clients and servers, including MSDOS, NetWare, UnixWare, OS/2 and most UNIX flavors.

History

Oracle began in 1977 and celebrating its 32 wonderful years in the industry (from 1977 to 2009).

1977 - Larry Ellison, Bob Miner and Ed Oates founded Software Development Laboratories to undertake development work.
1979 - Version 2.0 of Oracle was released and it became first commercial relational database and first SQL database. The company changed its name to Relational Software Inc. (RSI).
1981 - RSI started developing tools for Oracle.
1982 - RSI was renamed to Oracle Corporation.
1983 - Oracle released version 3.0, rewritten in C language and ran on multiple platforms.
1984 - Oracle version 4.0 was released. It contained features like concurrency control - multi-version read consistency, etc.
1985 - Oracle version 4.0 was released. It contained features like concurrency control - multi-version read consistency, etc.
2007 - Oracle released Oracle11g. The new version focused on better partitioning, easy migration, etc.

Features

Concurrency
Read Consistency
Locking Mechanisms
Quiesce Database
Portability
Self-managing database
SQL*Plus
ASM
Scheduler
Resource Manager
Data Warehousing
Materialized views
Bitmap indexes
Table compression
Parallel Execution
Analytic SQL
Data mining
Partitioning

MS ACCESS

This is one of the most popular Microsoft products. Microsoft Access is an entry-level database management software. MS Access database is not only inexpensive but also a powerful database for small-scale projects.

MS Access uses the Jet database engine, which utilizes a specific SQL language dialect (sometimes referred to as Jet SQL).

MS Access comes with the professional edition of MS Office package. MS Access has easyto-use intuitive graphical interface.

1992 - Access version 1.0 was released.
1993 - Access 1.1 released to improve compatibility with inclusion the Access Basic programming language.
The most significant transition was from Access 97 to Access 2000.
2007 - Access 2007, a new database format was introduced ACCDB which supports complex data types such as multi valued and attachment fields.

Features

Users can create tables, queries, forms and reports and connect them together with macros.
Option of importing and exporting the data to many formats including Excel, Outlook, ASCII, dBase, Paradox, FoxPro, SQL Server, Oracle, ODBC, etc.
There is also the Jet Database format (MDB or ACCDB in Access 2007), which can contain the application and data in one file. This makes it very convenient to distribute the entire application to another user, who can run it in disconnected environments.
Microsoft Access offers parameterized queries. These queries and Access tables can be referenced from other programs like VB6 and .NET through DAO or ADO.
The desktop editions of Microsoft SQL Server can be used with Access as an alternative to the Jet Database Engine.
Microsoft Access is a file server-based database. Unlike the client-server relational database management systems (RDBMS), Microsoft Access does not implement database triggers, stored procedures or transaction logging.

SQL - Syntax

SQL is followed by a unique set of rules and guidelines called Syntax. This tutorial gives you a quick start with SQL by listing all the basic SQL Syntax.

All the SQL statements start with any of the keywords like SELECT, INSERT, UPDATE, DELETE, ALTER, DROP, CREATE, USE, SHOW and all the statements end with a semicolon (;).

The most important point to be noted here is that SQL is case insensitive, which means SELECT and select have same meaning in SQL statements. Whereas, MySQL makes difference in table names. So, if you are working with MySQL, then you need to give table names as they exist in the database.

Various Syntax in SQL

All the examples given in this tutorial have been tested with a MySQL server.

SQL SELECT Statement

SQL CODE

SELECT column1, column2....columnN
FROM   table_name;

SQL DISTINCT Clause

SQL CODE

SELECT DISTINCT column1, column2....columnN
FROM   table_name;

SQL WHERE Clause

SQL CODE

SELECT column1, column2....columnN
FROM   table_name
WHERE  CONDITION;

SQL AND/OR Clause

SQL CODE

SELECT column1, column2....columnN
FROM   table_name
WHERE  CONDITION-1 {AND|OR} CONDITION-2;

SQL IN Clause

SQL CODE

SELECT column1, column2....columnN
FROM   table_name
WHERE  column_name IN (val-1, val-2,...val-N);

SQL BETWEEN Clause

SQL CODE

SELECT column1, column2....columnN
FROM   table_name
WHERE  column_name BETWEEN val-1 AND val-2;

SQL LIKE Clause

SQL CODE

SELECT column1, column2....columnN
FROM   table_name
WHERE  column_name LIKE { PATTERN };

SQL ORDER BY Clause

SQL CODE

SELECT column1, column2....columnN
FROM   table_name
WHERE  CONDITION
ORDER BY column_name {ASC|DESC};

SQL GROUP BY Clause

SQL CODE

SELECT SUM(column_name)
FROM   table_name
WHERE  CONDITION
GROUP BY column_name;

SQL COUNT Clause

SQL CODE

SELECT COUNT(column_name)
FROM   table_name
WHERE  CONDITION;

SQL HAVING Clause

SQL CODE

SELECT SUM(column_name)
FROM   table_name
WHERE  CONDITION
GROUP BY column_name
HAVING (arithematic function condition);

SQL CREATE TABLE Statement

SQL CODE

CREATE TABLE table_name(
column1 datatype,
column2 datatype,
column3 datatype,
.....
columnN datatype,
PRIMARY KEY( one or more columns )
);

SQL DROP TABLE Statement

SQL CODE

DROP TABLE table_name;

SQL CREATE INDEX Statement

SQL CODE

CREATE UNIQUE INDEX index_name
ON table_name ( column1, column2,...columnN);

SQL DROP INDEX Statement

SQL CODE

ALTER TABLE table_name
DROP INDEX index_name;

SQL DESC Statement

SQL CODE

DESC table_name;

SQL TRUNCATE TABLE Statement

SQL CODE

TRUNCATE TABLE table_name;

SQL ALTER TABLE Statement

SQL CODE

ALTER TABLE table_name {ADD|DROP|MODIFY} column_name {data_ype};

SQL ALTER TABLE Statement (Rename)

SQL CODE

ALTER TABLE table_name RENAME TO new_table_name;

SQL INSERT INTO Statement

SQL CODE

INSERT INTO table_name( column1, column2....columnN)
VALUES ( value1, value2....valueN);

SQL UPDATE Statement

SQL CODE

UPDATE table_name
SET column1 = value1, column2 = value2....columnN=valueN
[ WHERE  CONDITION ];

SQL DELETE Statement

SQL CODE

DELETE FROM table_name
WHERE  {CONDITION};

SQL CREATE DATABASE Statement

SQL CODE

CREATE DATABASE database_name;

SQL DROP DATABASE Statement

SQL CODE

DROP DATABASE database_name;

SQL USE Statement

SQL CODE

USE database_name;

SQL COMMIT Statement

SQL CODE

COMMIT;

SQL ROLLBACK Statement

SQL CODE

ROLLBACK;

SQL - Data Types

SQL Data Type is an attribute that specifies the type of data of any object. Each column, variable and expression has a related data type in SQL. You can use these data types while creating your tables. You can choose a data type for a table column based on your requirement.

SQL Server offers six categories of data types for your use which are listed below −

Exact Numeric Data Types

DATA TYPEFROMTObigint-9,223,372,036,854,775,8089,223,372,036,854,775,807int-2,147,483,6482,147,483,647smallint-32,76832,767tinyint0255bit01decimal-10^38 +110^38 -1numeric-10^38 +110^38 -1money-922,337,203,685,477.5808+922,337,203,685,477.5807smallmoney-214,748.3648+214,748.3647

Approximate Numeric Data Types

DATA TYPEFROMTOfloat-1.79E + 3081.79E + 308real-3.40E + 383.40E + 38

Date and Time Data Types

DATA TYPEFROMTOdatetimeJan 1, 1753Dec 31, 9999smalldatetimeJan 1, 1900Jun 6, 2079dateStores a date like June 30, 1991timeStores a time of day like 12:30 P.M.

Note− Here, datetime has 3.33 milliseconds accuracy where as smalldatetime has 1 minute accuracy.

Character Strings Data Types

Sr.No.DATA TYPE & Description1charMaximum length of 8,000 characters.( Fixed length non-Unicode characters)2varcharMaximum of 8,000 characters.(Variable-length non-Unicode data).3varchar(max)Maximum length of 2E + 31 characters, Variable-length non-Unicode data (SQL Server 2005 only).4textVariable-length non-Unicode data with a maximum length of 2,147,483,647 characters.

Unicode Character Strings Data Types

Sr.No.DATA TYPE & Description1ncharMaximum length of 4,000 characters.( Fixed length Unicode)2nvarcharMaximum length of 4,000 characters.(Variable length Unicode)3nvarchar(max)Maximum length of 2E + 31 characters (SQL Server 2005 only).( Variable length Unicode)4ntextMaximum length of 1,073,741,823 characters. ( Variable length Unicode )

Binary Data Types

Sr.No.DATA TYPE & Description1binaryMaximum length of 8,000 bytes(Fixed-length binary data )2varbinaryMaximum length of 8,000 bytes.(Variable length binary data)3varbinary(max)Maximum length of 2E + 31 bytes (SQL Server 2005 only). ( Variable length Binary data)4imageMaximum length of 2,147,483,647 bytes. ( Variable length Binary Data)

Misc Data Types

Sr.No.DATA TYPE & Description1sql_variantStores values of various SQL Server-supported data types, except text, ntext, and timestamp.2timestampStores a database-wide unique number that gets updated every time a row gets updated3uniqueidentifierStores a globally unique identifier (GUID)4xmlStores XML data. You can store xml instances in a column or a variable (SQL Server 2005 only).5cursorReference to a cursor object6tableStores a result set for later processing

SQL - Operators

What is an Operator in SQL?

An operator is a reserved word or a character used primarily in an SQL statement’s WHERE clause to perform operation(s), such as comparisons and arithmetic operations. These Operators are used to specify conditions in an SQL statement and to serve as conjunctions for multiple conditions in a statement.

Arithmetic operators
Comparison operators
Logical operators
Operators used to negate conditions

SQL Arithmetic Operators

Assume’variable a’holds 10 and’variable b’holds 20, then −

Show Examples

OperatorDescriptionExample+ (Addition)Adds values on either side of the operator.a + b will give 30- (Subtraction)Subtracts right hand operand from left hand operand.a - b will give -10* (Multiplication)Multiplies values on either side of the operator.a * b will give 200/ (Division)Divides left hand operand by right hand operand.b / a will give 2% (Modulus)Divides left hand operand by right hand operand and returns remainder.b % a will give 0

SQL Comparison Operators

Assume’variable a’holds 10 and’variable b’holds 20, then −

Show Examples

Operator	Description	Example
`=`	Checks if the values of two operands are equal or not	`(a = b)` is not true.
`!=`	Checks if the values of two operands are equal or not	`(a != b)` is true.
`<>`	Checks if the values of two operands are equal or not	`(a <> b)` is true.
`>`	Checks if the value of left operand is greater than right	`(a > b)` is not true.
`<`	Checks if the value of left operand is less than right	`(a < b)` is true.
`>=`	Checks if the value of left operand is greater than or equal	`(a >= b)` is not true.
`<=`	Checks if the value of left operand is less than or equal	`(a <= b)` is true.
`!<`	Checks if the value of left operand is not less than right	`(a !< b)` is false.
`!>`	Checks if the value of left operand is not greater than right	`(a !> b)` is true.

SQL Logical Operators

Here is a list of all the logical operators available in SQL.

Show Examples

Sr.No.Operator & Description1ALLThe ALL operator is used to compare a value to all values in another value set.2ANDThe AND operator allows the existence of multiple conditions in an SQL statement’s WHERE clause.3ANYThe ANY operator is used to compare a value to any applicable value in the list as per the condition.4BETWEENThe BETWEEN operator is used to search for values that are within a set of values, given the minimum value and the maximum value.5EXISTSThe EXISTS operator is used to search for the presence of a row in a specified table that meets a certain criterion.6INThe IN operator is used to compare a value to a list of literal values that have been specified.7LIKEThe LIKE operator is used to compare a value to similar values using wildcard operators.8NOTThe NOT operator reverses the meaning of the logical operator with which it is used. Eg: NOT EXISTS, NOT BETWEEN, NOT IN, etc.This is a negate operator.9ORThe OR operator is used to combine multiple conditions in an SQL statement’s WHERE clause.10IS NULLThe NULL operator is used to compare a value with a NULL value.11UNIQUEThe UNIQUE operator searches every row of a specified table for uniqueness (no duplicates).

SQL - Expressions

An expression is a combination of one or more values, operators and SQL functions that evaluate to a value. These SQL EXPRESSIONs are like formulae and they are written in query language. You can also use them to query the database for a specific set of data.

Syntax

Consider the basic syntax of the SELECT statement as follows −

SQL CODE

SELECT column1, column2, columnN 
FROM table_name 
WHERE [CONDITION|EXPRESSION];

There are different types of SQL expressions, which are mentioned below −

Boolean
Numeric
Date

Let us now discuss each of these in detail.

Boolean Expressions

SQL Boolean Expressions fetch the data based on matching a single value. Following is the syntax −

SQL CODE

SELECT column1, column2, columnN 
FROM table_name 
WHERE SINGLE VALUE MATCHING EXPRESSION;

Consider the CUSTOMERS table having the following records −

SQL CODE

SQL&gt; SELECT * FROM CUSTOMERS;
+----+----------+-----+-----------+----------+
| ID | NAME     | AGE | ADDRESS   | SALARY   |
+----+----------+-----+-----------+----------+
|  1 | Ramesh   |  32 | Ahmedabad |  2000.00 |
|  2 | Khilan   |  25 | Delhi     |  1500.00 |
|  3 | kaushik  |  23 | Kota      |  2000.00 |
|  4 | Chaitali |  25 | Mumbai    |  6500.00 |
|  5 | Hardik   |  27 | Bhopal    |  8500.00 |
|  6 | Komal    |  22 | MP        |  4500.00 |
|  7 | Muffy    |  24 | Indore    | 10000.00 |
+----+----------+-----+-----------+----------+
7 rows in set (0.00 sec)

The following table is a simple example showing the usage of various SQL Boolean Expressions −

SQL CODE

SQL&gt; SELECT * FROM CUSTOMERS WHERE SALARY = 10000;
+----+-------+-----+---------+----------+
| ID | NAME  | AGE | ADDRESS | SALARY   |
+----+-------+-----+---------+----------+
|  7 | Muffy |  24 | Indore  | 10000.00 |
+----+-------+-----+---------+----------+
1 row in set (0.00 sec)

Numeric Expression

These expressions are used to perform any mathematical operation in any query. Following is the syntax −

SQL CODE

SELECT numerical_expression as  OPERATION_NAME
[FROM table_name
WHERE CONDITION] ;

Here, the numerical_expression is used for a mathematical expression or any formula. Following is a simple example showing the usage of SQL Numeric Expressions −

SQL CODE

SQL&gt; SELECT (15 + 6) AS ADDITION
+----------+
| ADDITION |
+----------+
|       21 |
+----------+
1 row in set (0.00 sec)

There are several built-in functions like avg(), sum(), count(), etc., to perform what is known as the aggregate data calculations against a table or a specific table column.

SQL CODE

SQL&gt; SELECT COUNT(*) AS "RECORDS" FROM CUSTOMERS; 
+---------+
| RECORDS |
+---------+
|       7 |
+---------+
1 row in set (0.00 sec)

Date Expressions

Date Expressions return current system date and time values −

SQL CODE

SQL&gt;  SELECT CURRENT_TIMESTAMP;
+---------------------+
| Current_Timestamp   |
+---------------------+
| 2009-11-12 06:40:23 |
+---------------------+
1 row in set (0.00 sec)

Another date expression is as shown below −

SQL CODE

SQL&gt;  SELECT  GETDATE();;
+-------------------------+
| GETDATE                 |
+-------------------------+
| 2009-10-22 12:07:18.140 |
+-------------------------+
1 row in set (0.00 sec)

SQL - CREATE Database

The SQLCREATE DATABASEstatement is used to create a new SQL database.

Syntax

The basic syntax of this CREATE DATABASE statement is as follows −

SQL CODE

CREATE DATABASE DatabaseName;

Always the database name should be unique within the RDBMS.

Example

If you want to create a new database <testDB>, then the CREATE DATABASE statement would be as shown below −

SQL CODE

SQL&gt; CREATE DATABASE testDB;

Make sure you have the admin privilege before creating any database. Once a database is created, you can check it in the list of databases as follows −

SQL CODE

SQL&gt; SHOW DATABASES;
+--------------------+
| Database           |
+--------------------+
| information_schema |
| AMROOD             |
| CODESCOMPILER     |
| mysql              |
| orig               |
| test               |
| testDB             |
+--------------------+
7 rows in set (0.00 sec)

SQL - DROP or DELETE Database

The SQLDROP DATABASEstatement is used to drop an existing database in SQL schema.

Syntax

The basic syntax of DROP DATABASE statement is as follows −

SQL CODE

DROP DATABASE DatabaseName;

Always the database name should be unique within the RDBMS.

Example

If you want to delete an existing database <testDB>, then the DROP DATABASE statement would be as shown below −

SQL CODE

SQL&gt; DROP DATABASE testDB;

NOTE− Be careful before using this operation because by deleting an existing database would result in loss of complete information stored in the database.

Make sure you have the admin privilege before dropping any database. Once a database is dropped, you can check it in the list of the databases as shown below −

SQL CODE

SQL&gt; SHOW DATABASES;
+--------------------+
| Database           |
+--------------------+
| information_schema |
| AMROOD             |
| CODESCOMPILER     |
| mysql              |
| orig               |
| test               |
+--------------------+
6 rows in set (0.00 sec)

SQL - SELECT Database, USE Statement

When you have multiple databases in your SQL Schema, then before starting your operation, you would need to select a database where all the operations would be performed.

The SQLUSEstatement is used to select any existing database in the SQL schema.

Syntax

The basic syntax of the USE statement is as shown below −

SQL CODE

USE DatabaseName;

Always the database name should be unique within the RDBMS.

Example

You can check the available databases as shown below −

SQL CODE

SQL&gt; SHOW DATABASES;
+--------------------+
| Database           |
+--------------------+
| information_schema |
| AMROOD             |
| CODESCOMPILER     |
| mysql              |
| orig               |
| test               |
+--------------------+
6 rows in set (0.00 sec)

Now, if you want to work with the AMROOD database, then you can execute the following SQL command and start working with the AMROOD database.

SQL CODE

SQL&gt; USE AMROOD;

SQL - CREATE Table

Creating a basic table involves naming the table and defining its columns and each column’s data type.

The SQLCREATE TABLEstatement is used to create a new table.

Syntax

The basic syntax of the CREATE TABLE statement is as follows −

SQL CODE

CREATE TABLE table_name(
   column1 datatype,
   column2 datatype,
   column3 datatype,
   .....
   columnN datatype,
   PRIMARY KEY( one or more columns )
);

CREATE TABLE is the keyword telling the database system what you want to do. In this case, you want to create a new table. The unique name or identifier for the table follows the CREATE TABLE statement.

Then in brackets comes the list defining each column in the table and what sort of data type it is. The syntax becomes clearer with the following example.

A copy of an existing table can be created using a combination of the CREATE TABLE statement and the SELECT statement. You can check the complete details atCreate Table Using another Table.

Example

The following code block is an example, which creates a CUSTOMERS table with an ID as a primary key and NOT NULL are the constraints showing that these fields cannot be NULL while creating records in this table −

SQL CODE

SQL&gt; CREATE TABLE CUSTOMERS(
   ID   INT              NOT NULL,
   NAME VARCHAR (20)     NOT NULL,
   AGE  INT              NOT NULL,
   ADDRESS  CHAR (25) ,
   SALARY   DECIMAL (18, 2),       
   PRIMARY KEY (ID)
);

You can verify if your table has been created successfully by looking at the message displayed by the SQL server, otherwise you can use theDESCcommand as follows −

SQL CODE

SQL&gt; DESC CUSTOMERS;
+---------+---------------+------+-----+---------+-------+
| Field   | Type          | Null | Key | Default | Extra |
+---------+---------------+------+-----+---------+-------+
| ID      | int(11)       | NO   | PRI |         |       |
| NAME    | varchar(20)   | NO   |     |         |       |
| AGE     | int(11)       | NO   |     |         |       |
| ADDRESS | char(25)      | YES  |     | NULL    |       |
| SALARY  | decimal(18,2) | YES  |     | NULL    |       |
+---------+---------------+------+-----+---------+-------+
5 rows in set (0.00 sec)

Now, you have CUSTOMERS table available in your database which you can use to store the required information related to customers.

SQL - DROP or DELETE Table

The SQLDROP TABLEstatement is used to remove a table definition and all the data, indexes, triggers, constraints and permission specifications for that table.

NOTE− You should be very careful while using this command because once a table is deleted then all the information available in that table will also be lost forever.

Syntax

The basic syntax of this DROP TABLE statement is as follows −

SQL CODE

DROP TABLE table_name;

Example

Let us first verify the CUSTOMERS table and then we will delete it from the database as shown below −

SQL CODE

SQL&gt; DESC CUSTOMERS;
+---------+---------------+------+-----+---------+-------+
| Field   | Type          | Null | Key | Default | Extra |
+---------+---------------+------+-----+---------+-------+
| ID      | int(11)       | NO   | PRI |         |       |
| NAME    | varchar(20)   | NO   |     |         |       |
| AGE     | int(11)       | NO   |     |         |       |
| ADDRESS | char(25)      | YES  |     | NULL    |       |
| SALARY  | decimal(18,2) | YES  |     | NULL    |       |
+---------+---------------+------+-----+---------+-------+
5 rows in set (0.00 sec)

This means that the CUSTOMERS table is available in the database, so let us now drop it as shown below.

SQL CODE

SQL&gt; DROP TABLE CUSTOMERS;
Query OK, 0 rows affected (0.01 sec)

Now, if you would try the DESC command, then you will get the following error −

SQL CODE

SQL&gt; DESC CUSTOMERS;
ERROR 1146 (42S02): Table 'TEST.CUSTOMERS' doesn't exist

Here, TEST is the database name which we are using for our examples.

SQL - INSERT Query

The SQLINSERT INTOStatement is used to add new rows of data to a table in the database.

Syntax

There are two basic syntaxes of the INSERT INTO statement which are shown below.

SQL CODE

INSERT INTO TABLE_NAME (column1, column2, column3,...columnN)  
VALUES (value1, value2, value3,...valueN);

Here, column1, column2, column3,…columnN are the names of the columns in the table into which you want to insert the data.

You may not need to specify the column(s) name in the SQL query if you are adding values for all the columns of the table. But make sure the order of the values is in the same order as the columns in the table.

TheSQL INSERT INTOsyntax will be as follows −

SQL CODE

INSERT INTO TABLE_NAME VALUES (value1,value2,value3,...valueN);

Example

The following statements would create six records in the CUSTOMERS table.

SQL CODE

INSERT INTO CUSTOMERS (ID,NAME,AGE,ADDRESS,SALARY)
VALUES (1, 'Ramesh', 32, 'Ahmedabad', 2000.00 );

INSERT INTO CUSTOMERS (ID,NAME,AGE,ADDRESS,SALARY)
VALUES (2, 'Khilan', 25, 'Delhi', 1500.00 );

INSERT INTO CUSTOMERS (ID,NAME,AGE,ADDRESS,SALARY)
VALUES (3, 'kaushik', 23, 'Kota', 2000.00 );

INSERT INTO CUSTOMERS (ID,NAME,AGE,ADDRESS,SALARY)
VALUES (4, 'Chaitali', 25, 'Mumbai', 6500.00 );

INSERT INTO CUSTOMERS (ID,NAME,AGE,ADDRESS,SALARY)
VALUES (5, 'Hardik', 27, 'Bhopal', 8500.00 );

INSERT INTO CUSTOMERS (ID,NAME,AGE,ADDRESS,SALARY)
VALUES (6, 'Komal', 22, 'MP', 4500.00 );

You can create a record in the CUSTOMERS table by using the second syntax as shown below.

SQL CODE

INSERT INTO CUSTOMERS 
VALUES (7, 'Muffy', 24, 'Indore', 10000.00 );

All the above statements would produce the following records in the CUSTOMERS table as shown below.

SQL CODE

+----+----------+-----+-----------+----------+
| ID | NAME     | AGE | ADDRESS   | SALARY   |
+----+----------+-----+-----------+----------+
|  1 | Ramesh   |  32 | Ahmedabad |  2000.00 |
|  2 | Khilan   |  25 | Delhi     |  1500.00 |
|  3 | kaushik  |  23 | Kota      |  2000.00 |
|  4 | Chaitali |  25 | Mumbai    |  6500.00 |
|  5 | Hardik   |  27 | Bhopal    |  8500.00 |
|  6 | Komal    |  22 | MP        |  4500.00 |
|  7 | Muffy    |  24 | Indore    | 10000.00 |
+----+----------+-----+-----------+----------+

Populate one table using another table

You can populate the data into a table through the select statement over another table; provided the other table has a set of fields, which are required to populate the first table.

Here is the syntax −

SQL CODE

INSERT INTO first_table_name [(column1, column2, ... columnN)] 
   SELECT column1, column2, ...columnN 
   FROM second_table_name
   [WHERE condition];

SQL - SELECT Query

The SQLSELECTstatement is used to fetch the data from a database table which returns this data in the form of a result table. These result tables are called result-sets.

Syntax

The basic syntax of the SELECT statement is as follows −

SQL CODE

SELECT column1, column2, columnN FROM table_name;

Here, column1, column2… are the fields of a table whose values you want to fetch. If you want to fetch all the fields available in the field, then you can use the following syntax.

SQL CODE

SELECT * FROM table_name;

Example

Consider the CUSTOMERS table having the following records −

SQL CODE

+----+----------+-----+-----------+----------+
| ID | NAME     | AGE | ADDRESS   | SALARY   |
+----+----------+-----+-----------+----------+
|  1 | Ramesh   |  32 | Ahmedabad |  2000.00 |
|  2 | Khilan   |  25 | Delhi     |  1500.00 |
|  3 | kaushik  |  23 | Kota      |  2000.00 |
|  4 | Chaitali |  25 | Mumbai    |  6500.00 |
|  5 | Hardik   |  27 | Bhopal    |  8500.00 |
|  6 | Komal    |  22 | MP        |  4500.00 |
|  7 | Muffy    |  24 | Indore    | 10000.00 |
+----+----------+-----+-----------+----------+

The following code is an example, which would fetch the ID, Name and Salary fields of the customers available in CUSTOMERS table.

SQL CODE

SQL&gt; SELECT ID, NAME, SALARY FROM CUSTOMERS;

This would produce the following result −

SQL CODE

+----+----------+----------+
| ID | NAME     | SALARY   |
+----+----------+----------+
|  1 | Ramesh   |  2000.00 |
|  2 | Khilan   |  1500.00 |
|  3 | kaushik  |  2000.00 |
|  4 | Chaitali |  6500.00 |
|  5 | Hardik   |  8500.00 |
|  6 | Komal    |  4500.00 |
|  7 | Muffy    | 10000.00 |
+----+----------+----------+

If you want to fetch all the fields of the CUSTOMERS table, then you should use the following query.

SQL CODE

SQL&gt; SELECT * FROM CUSTOMERS;

This would produce the result as shown below.

SQL CODE

+----+----------+-----+-----------+----------+
| ID | NAME     | AGE | ADDRESS   | SALARY   |
+----+----------+-----+-----------+----------+
|  1 | Ramesh   |  32 | Ahmedabad |  2000.00 |
|  2 | Khilan   |  25 | Delhi     |  1500.00 |
|  3 | kaushik  |  23 | Kota      |  2000.00 |
|  4 | Chaitali |  25 | Mumbai    |  6500.00 |
|  5 | Hardik   |  27 | Bhopal    |  8500.00 |
|  6 | Komal    |  22 | MP        |  4500.00 |
|  7 | Muffy    |  24 | Indore    | 10000.00 |
+----+----------+-----+-----------+----------+

SQL - WHERE Clause

The SQLWHEREclause is used to specify a condition while fetching the data from a single table or by joining with multiple tables. If the given condition is satisfied, then only it returns a specific value from the table. You should use the WHERE clause to filter the records and fetching only the necessary records.

The WHERE clause is not only used in the SELECT statement, but it is also used in the UPDATE, DELETE statement, etc., which we would examine in the subsequent chapters.

Syntax

The basic syntax of the SELECT statement with the WHERE clause is as shown below.

SQL CODE

SELECT column1, column2, columnN 
FROM table_name
WHERE [condition]

You can specify a condition using thecomparison or logical operatorslike >, <, =,LIKE, NOT, etc. The following examples would make this concept clear.

Example

Consider the CUSTOMERS table having the following records −

SQL CODE

+----+----------+-----+-----------+----------+
| ID | NAME     | AGE | ADDRESS   | SALARY   |
+----+----------+-----+-----------+----------+
|  1 | Ramesh   |  32 | Ahmedabad |  2000.00 |
|  2 | Khilan   |  25 | Delhi     |  1500.00 |
|  3 | kaushik  |  23 | Kota      |  2000.00 |
|  4 | Chaitali |  25 | Mumbai    |  6500.00 |
|  5 | Hardik   |  27 | Bhopal    |  8500.00 |
|  6 | Komal    |  22 | MP        |  4500.00 |
|  7 | Muffy    |  24 | Indore    | 10000.00 |
+----+----------+-----+-----------+----------+

The following code is an example which would fetch the ID, Name and Salary fields from the CUSTOMERS table, where the salary is greater than 2000 −

SQL CODE

SQL&gt; SELECT ID, NAME, SALARY 
FROM CUSTOMERS
WHERE SALARY &gt; 2000;

This would produce the following result −

SQL CODE

+----+----------+----------+
| ID | NAME     | SALARY   |
+----+----------+----------+
|  4 | Chaitali |  6500.00 |
|  5 | Hardik   |  8500.00 |
|  6 | Komal    |  4500.00 |
|  7 | Muffy    | 10000.00 |
+----+----------+----------+

The following query is an example, which would fetch the ID, Name and Salary fields from the CUSTOMERS table for a customer with the nameHardik.

Here, it is important to note that all the strings should be given inside single quotes (”). Whereas, numeric values should be given without any quote as in the above example.

SQL CODE

SQL&gt; SELECT ID, NAME, SALARY 
FROM CUSTOMERS
WHERE NAME = 'Hardik';

This would produce the following result −

SQL CODE

+----+----------+----------+
| ID | NAME     | SALARY   |
+----+----------+----------+
|  5 | Hardik   |  8500.00 |
+----+----------+----------+

SQL - AND and OR Conjunctive Operators

The SQLAND&ORoperators are used to combine multiple conditions to narrow data in an SQL statement. These two operators are called as the conjunctive operators.

These operators provide a means to make multiple comparisons with different operators in the same SQL statement.

The AND Operator

TheANDoperator allows the existence of multiple conditions in an SQL statement’s WHERE clause.

Syntax

The basic syntax of the AND operator with a WHERE clause is as follows −

SQL CODE

SELECT column1, column2, columnN 
FROM table_name
WHERE [condition1] AND [condition2]...AND [conditionN];

You can combine N number of conditions using the AND operator. For an action to be taken by the SQL statement, whether it be a transaction or a query, all conditions separated by the AND must be TRUE.

Example

Consider the CUSTOMERS table having the following records −

SQL CODE

+----+----------+-----+-----------+----------+
| ID | NAME     | AGE | ADDRESS   | SALARY   |
+----+----------+-----+-----------+----------+
|  1 | Ramesh   |  32 | Ahmedabad |  2000.00 |
|  2 | Khilan   |  25 | Delhi     |  1500.00 |
|  3 | kaushik  |  23 | Kota      |  2000.00 |
|  4 | Chaitali |  25 | Mumbai    |  6500.00 |
|  5 | Hardik   |  27 | Bhopal    |  8500.00 |
|  6 | Komal    |  22 | MP        |  4500.00 |
|  7 | Muffy    |  24 | Indore    | 10000.00 |
+----+----------+-----+-----------+----------+

Following is an example, which would fetch the ID, Name and Salary fields from the CUSTOMERS table, where the salary is greater than 2000 and the age is less than 25 years −

SQL CODE

SQL&gt; SELECT ID, NAME, SALARY 
FROM CUSTOMERS
WHERE SALARY &gt; 2000 AND age &lt; 25;

This would produce the following result −

SQL CODE

+----+-------+----------+
| ID | NAME  | SALARY   |
+----+-------+----------+
|  6 | Komal |  4500.00 |
|  7 | Muffy | 10000.00 |
+----+-------+----------+

The OR Operator

The OR operator is used to combine multiple conditions in an SQL statement’s WHERE clause.

Syntax

The basic syntax of the OR operator with a WHERE clause is as follows −

SQL CODE

SELECT column1, column2, columnN 
FROM table_name
WHERE [condition1] OR [condition2]...OR [conditionN]

You can combine N number of conditions using the OR operator. For an action to be taken by the SQL statement, whether it be a transaction or query, the only any ONE of the conditions separated by the OR must be TRUE.

Example

Consider the CUSTOMERS table having the following records −

SQL CODE

+----+----------+-----+-----------+----------+
| ID | NAME     | AGE | ADDRESS   | SALARY   |
+----+----------+-----+-----------+----------+
|  1 | Ramesh   |  32 | Ahmedabad |  2000.00 |
|  2 | Khilan   |  25 | Delhi     |  1500.00 |
|  3 | kaushik  |  23 | Kota      |  2000.00 |
|  4 | Chaitali |  25 | Mumbai    |  6500.00 |
|  5 | Hardik   |  27 | Bhopal    |  8500.00 |
|  6 | Komal    |  22 | MP        |  4500.00 |
|  7 | Muffy    |  24 | Indore    | 10000.00 |
+----+----------+-----+-----------+----------+

The following code block hasa query, which would fetch the ID, Name and Salary fields from the CUSTOMERS table, where the salary is greater than 2000 OR the age is less than 25 years.

SQL CODE

SQL&gt; SELECT ID, NAME, SALARY 
FROM CUSTOMERS
WHERE SALARY &gt; 2000 OR age &lt; 25;

This would produce the following result −

SQL CODE

+----+----------+----------+
| ID | NAME     | SALARY   |
+----+----------+----------+
|  3 | kaushik  |  2000.00 |
|  4 | Chaitali |  6500.00 |
|  5 | Hardik   |  8500.00 |
|  6 | Komal    |  4500.00 |
|  7 | Muffy    | 10000.00 |
+----+----------+----------+

SQL - UPDATE Query

The SQLUPDATEQuery is used to modify the existing records in a table. You can use the WHERE clause with the UPDATE query to update the selected rows, otherwise all the rows would be affected.

Syntax

The basic syntax of the UPDATE query with a WHERE clause is as follows −

SQL CODE

UPDATE table_name
SET column1 = value1, column2 = value2...., columnN = valueN
WHERE [condition];

You can combine N number of conditions using the AND or the OR operators.

Example

Consider the CUSTOMERS table having the following records −

SQL CODE

+----+----------+-----+-----------+----------+
| ID | NAME     | AGE | ADDRESS   | SALARY   |
+----+----------+-----+-----------+----------+
|  1 | Ramesh   |  32 | Ahmedabad |  2000.00 |
|  2 | Khilan   |  25 | Delhi     |  1500.00 |
|  3 | kaushik  |  23 | Kota      |  2000.00 |
|  4 | Chaitali |  25 | Mumbai    |  6500.00 |
|  5 | Hardik   |  27 | Bhopal    |  8500.00 |
|  6 | Komal    |  22 | MP        |  4500.00 |
|  7 | Muffy    |  24 | Indore    | 10000.00 |
+----+----------+-----+-----------+----------+

The following query will update the ADDRESS for a customer whose ID number is 6 in the table.

SQL CODE

SQL&gt; UPDATE CUSTOMERS
SET ADDRESS = 'Pune'
WHERE ID = 6;

Now, the CUSTOMERS table would have the following records −

SQL CODE

+----+----------+-----+-----------+----------+
| ID | NAME     | AGE | ADDRESS   | SALARY   |
+----+----------+-----+-----------+----------+
|  1 | Ramesh   |  32 | Ahmedabad |  2000.00 |
|  2 | Khilan   |  25 | Delhi     |  1500.00 |
|  3 | kaushik  |  23 | Kota      |  2000.00 |
|  4 | Chaitali |  25 | Mumbai    |  6500.00 |
|  5 | Hardik   |  27 | Bhopal    |  8500.00 |
|  6 | Komal    |  22 | Pune      |  4500.00 |
|  7 | Muffy    |  24 | Indore    | 10000.00 |
+----+----------+-----+-----------+----------+

If you want to modify all the ADDRESS and the SALARY column values in the CUSTOMERS table, you do not need to use the WHERE clause as the UPDATE query would be enough as shown in the following code block.

SQL CODE

SQL&gt; UPDATE CUSTOMERS
SET ADDRESS = 'Pune', SALARY = 1000.00;

Now, CUSTOMERS table would have the following records −

SQL CODE

+----+----------+-----+---------+---------+
| ID | NAME     | AGE | ADDRESS | SALARY  |
+----+----------+-----+---------+---------+
|  1 | Ramesh   |  32 | Pune    | 1000.00 |
|  2 | Khilan   |  25 | Pune    | 1000.00 |
|  3 | kaushik  |  23 | Pune    | 1000.00 |
|  4 | Chaitali |  25 | Pune    | 1000.00 |
|  5 | Hardik   |  27 | Pune    | 1000.00 |
|  6 | Komal    |  22 | Pune    | 1000.00 |
|  7 | Muffy    |  24 | Pune    | 1000.00 |
+----+----------+-----+---------+---------+

SQL - DELETE Query

The SQL DELETE Query is used to delete the existing records from a table.

You can use the WHERE clause with a DELETE query to delete the selected rows, otherwise all the records would be deleted.

Syntax

The basic syntax of the DELETE query with the WHERE clause is as follows −

SQL CODE

DELETE FROM table_name
WHERE [condition];

You can combine N number of conditions using AND or OR operators.

Example

Consider the CUSTOMERS table having the following records −

SQL CODE

+----+----------+-----+-----------+----------+
| ID | NAME     | AGE | ADDRESS   | SALARY   |
+----+----------+-----+-----------+----------+
|  1 | Ramesh   |  32 | Ahmedabad |  2000.00 |
|  2 | Khilan   |  25 | Delhi     |  1500.00 |
|  3 | kaushik  |  23 | Kota      |  2000.00 |
|  4 | Chaitali |  25 | Mumbai    |  6500.00 |
|  5 | Hardik   |  27 | Bhopal    |  8500.00 |
|  6 | Komal    |  22 | MP        |  4500.00 |
|  7 | Muffy    |  24 | Indore    | 10000.00 |
+----+----------+-----+-----------+----------+

The following code has a query, which will DELETE a customer, whose ID is 6.

SQL CODE

SQL&gt; DELETE FROM CUSTOMERS
WHERE ID = 6;

Now, the CUSTOMERS table would have the following records.

SQL CODE

+----+----------+-----+-----------+----------+
| ID | NAME     | AGE | ADDRESS   | SALARY   |
+----+----------+-----+-----------+----------+
|  1 | Ramesh   |  32 | Ahmedabad |  2000.00 |
|  2 | Khilan   |  25 | Delhi     |  1500.00 |
|  3 | kaushik  |  23 | Kota      |  2000.00 |
|  4 | Chaitali |  25 | Mumbai    |  6500.00 |
|  5 | Hardik   |  27 | Bhopal    |  8500.00 |
|  7 | Muffy    |  24 | Indore    | 10000.00 |
+----+----------+-----+-----------+----------+

If you want to DELETE all the records from the CUSTOMERS table, you do not need to use the WHERE clause and the DELETE query would be as follows −

SQL CODE

SQL&gt; DELETE FROM CUSTOMERS;

Now, the CUSTOMERS table would not have any record.

SQL - LIKE Clause

The SQLLIKEclause is used to compare a value to similar values using wildcard operators. There are two wildcards used in conjunction with the LIKE operator.

The percent sign (%)
The underscore (_)

The percent sign represents zero, one or multiple characters. The underscore represents a single number or character. These symbols can be used in combinations.

Syntax

The basic syntax of % and _ is as follows −

SQL CODE

SELECT FROM table_name
WHERE column LIKE 'XXXX%'

or 

SELECT FROM table_name
WHERE column LIKE '%XXXX%'

or

SELECT FROM table_name
WHERE column LIKE 'XXXX_'

or

SELECT FROM table_name
WHERE column LIKE '_XXXX'

or

SELECT FROM table_name
WHERE column LIKE '_XXXX_'

You can combine N number of conditions using AND or OR operators. Here, XXXX could be any numeric or string value.

Example

The following table has a few examples showing the WHERE part having different LIKE clause with ’%’ and ’_’ operators −

Sr.No.Statement & Description1WHERE SALARY LIKE ‘200%‘Finds any values that start with 200.2WHERE SALARY LIKE ‘%200%‘Finds any values that have 200 in any position.3WHERE SALARY LIKE ‘00%‘Finds any values that have 00 in the second and third positions.4WHERE SALARY LIKE ‘2%_%‘Finds any values that start with 2 and are at least 3 characters in length.5WHERE SALARY LIKE ‘%2’Finds any values that end with 2.6WHERE SALARY LIKE ‘_2%3’Finds any values that have a 2 in the second position and end with a 3.7WHERE SALARY LIKE ‘2___3’Finds any values in a five-digit number that start with 2 and end with 3.

Let us take a real example, consider the CUSTOMERS table having the records as shown below.

SQL CODE

+----+----------+-----+-----------+----------+
| ID | NAME     | AGE | ADDRESS   | SALARY   |
+----+----------+-----+-----------+----------+
|  1 | Ramesh   |  32 | Ahmedabad |  2000.00 |
|  2 | Khilan   |  25 | Delhi     |  1500.00 |
|  3 | kaushik  |  23 | Kota      |  2000.00 |
|  4 | Chaitali |  25 | Mumbai    |  6500.00 |
|  5 | Hardik   |  27 | Bhopal    |  8500.00 |
|  6 | Komal    |  22 | MP        |  4500.00 |
|  7 | Muffy    |  24 | Indore    | 10000.00 |
+----+----------+-----+-----------+----------+

Following is an example, which would display all the records from the CUSTOMERS table, where the SALARY starts with 200.

SQL CODE

SQL&gt; SELECT * FROM CUSTOMERS
WHERE SALARY LIKE '200%';

This would produce the following result −

SQL CODE

+----+----------+-----+-----------+----------+
| ID | NAME     | AGE | ADDRESS   | SALARY   |
+----+----------+-----+-----------+----------+
|  1 | Ramesh   |  32 | Ahmedabad |  2000.00 |
|  3 | kaushik  |  23 | Kota      |  2000.00 |
+----+----------+-----+-----------+----------+

SQL - TOP, LIMIT or ROWNUM Clause

The SQLTOPclause is used to fetch a TOP N number or X percent records from a table.

Note− All the databases do not support the TOP clause. For example MySQL supports theLIMITclause to fetch limited number of records while Oracle uses theROWNUMcommand to fetch a limited number of records.

Syntax

The basic syntax of the TOP clause with a SELECT statement would be as follows.

SQL CODE

SELECT TOP number|percent column_name(s)
FROM table_name
WHERE [condition]

Example

Consider the CUSTOMERS table having the following records −

SQL CODE

+----+----------+-----+-----------+----------+
| ID | NAME     | AGE | ADDRESS   | SALARY   |
+----+----------+-----+-----------+----------+
|  1 | Ramesh   |  32 | Ahmedabad |  2000.00 |
|  2 | Khilan   |  25 | Delhi     |  1500.00 |
|  3 | kaushik  |  23 | Kota      |  2000.00 |
|  4 | Chaitali |  25 | Mumbai    |  6500.00 |
|  5 | Hardik   |  27 | Bhopal    |  8500.00 |
|  6 | Komal    |  22 | MP        |  4500.00 |
|  7 | Muffy    |  24 | Indore    | 10000.00 |
+----+----------+-----+-----------+----------+

The following query is an example on the SQL server, which would fetch the top 3 records from the CUSTOMERS table.

SQL CODE

SQL&gt; SELECT TOP 3 * FROM CUSTOMERS;

This would produce the following result −

SQL CODE

+----+---------+-----+-----------+---------+
| ID | NAME    | AGE | ADDRESS   | SALARY  |
+----+---------+-----+-----------+---------+
|  1 | Ramesh  |  32 | Ahmedabad | 2000.00 |
|  2 | Khilan  |  25 | Delhi     | 1500.00 |
|  3 | kaushik |  23 | Kota      | 2000.00 |
+----+---------+-----+-----------+---------+

If you are using MySQL server, then here is an equivalent example −

SQL CODE

SQL&gt; SELECT * FROM CUSTOMERS
LIMIT 3;

This would produce the following result −

SQL CODE

+----+---------+-----+-----------+---------+
| ID | NAME    | AGE | ADDRESS   | SALARY  |
+----+---------+-----+-----------+---------+
|  1 | Ramesh  |  32 | Ahmedabad | 2000.00 |
|  2 | Khilan  |  25 | Delhi     | 1500.00 |
|  3 | kaushik |  23 | Kota      | 2000.00 |
+----+---------+-----+-----------+---------+

If you are using an Oracle server, then the following code block has an equivalent example.

SQL CODE

SQL&gt; SELECT * FROM CUSTOMERS
WHERE ROWNUM &lt;= 3;

This would produce the following result −

SQL CODE

+----+---------+-----+-----------+---------+
| ID | NAME    | AGE | ADDRESS   | SALARY  |
+----+---------+-----+-----------+---------+
|  1 | Ramesh  |  32 | Ahmedabad | 2000.00 |
|  2 | Khilan  |  25 | Delhi     | 1500.00 |
|  3 | kaushik |  23 | Kota      | 2000.00 |
+----+---------+-----+-----------+---------+

SQL - ORDER BY Clause

The SQLORDER BYclause is used to sort the data in ascending or descending order, based on one or more columns. Some databases sort the query results in an ascending order by default.

Syntax

The basic syntax of the ORDER BY clause is as follows −

SQL CODE

SELECT column-list 
FROM table_name 
[WHERE condition] 
[ORDER BY column1, column2, .. columnN] [ASC | DESC];

You can use more than one column in the ORDER BY clause. Make sure whatever column you are using to sort that column should be in the column-list.

Example

Consider the CUSTOMERS table having the following records −

SQL CODE

+----+----------+-----+-----------+----------+
| ID | NAME     | AGE | ADDRESS   | SALARY   |
+----+----------+-----+-----------+----------+
|  1 | Ramesh   |  32 | Ahmedabad |  2000.00 |
|  2 | Khilan   |  25 | Delhi     |  1500.00 |
|  3 | kaushik  |  23 | Kota      |  2000.00 |
|  4 | Chaitali |  25 | Mumbai    |  6500.00 |
|  5 | Hardik   |  27 | Bhopal    |  8500.00 |
|  6 | Komal    |  22 | MP        |  4500.00 |
|  7 | Muffy    |  24 | Indore    | 10000.00 |
+----+----------+-----+-----------+----------+

The following code block has an example, which would sort the result in an ascending order by the NAME and the SALARY −

SQL CODE

SQL&gt; SELECT * FROM CUSTOMERS
   ORDER BY NAME, SALARY;

This would produce the following result −

SQL CODE

+----+----------+-----+-----------+----------+
| ID | NAME     | AGE | ADDRESS   | SALARY   |
+----+----------+-----+-----------+----------+
|  4 | Chaitali |  25 | Mumbai    |  6500.00 |
|  5 | Hardik   |  27 | Bhopal    |  8500.00 |
|  3 | kaushik  |  23 | Kota      |  2000.00 |
|  2 | Khilan   |  25 | Delhi     |  1500.00 |
|  6 | Komal    |  22 | MP        |  4500.00 |
|  7 | Muffy    |  24 | Indore    | 10000.00 |
|  1 | Ramesh   |  32 | Ahmedabad |  2000.00 |
+----+----------+-----+-----------+----------+

The following code block has an example, which would sort the result in the descending order by NAME.

SQL CODE

SQL&gt; SELECT * FROM CUSTOMERS
   ORDER BY NAME DESC;

This would produce the following result −

SQL CODE

+----+----------+-----+-----------+----------+
| ID | NAME     | AGE | ADDRESS   | SALARY   |
+----+----------+-----+-----------+----------+
|  1 | Ramesh   |  32 | Ahmedabad |  2000.00 |
|  7 | Muffy    |  24 | Indore    | 10000.00 |
|  6 | Komal    |  22 | MP        |  4500.00 |
|  2 | Khilan   |  25 | Delhi     |  1500.00 |
|  3 | kaushik  |  23 | Kota      |  2000.00 |
|  5 | Hardik   |  27 | Bhopal    |  8500.00 |
|  4 | Chaitali |  25 | Mumbai    |  6500.00 |
+----+----------+-----+-----------+----------+

SQL - Group By

The SQLGROUP BYclause is used in collaboration with the SELECT statement to arrange identical data into groups. This GROUP BY clause follows the WHERE clause in a SELECT statement and precedes the ORDER BY clause.

Syntax

The basic syntax of a GROUP BY clause is shown in the following code block. The GROUP BY clause must follow the conditions in the WHERE clause and must precede the ORDER BY clause if one is used.

SQL CODE

SELECT column1, column2
FROM table_name
WHERE [ conditions ]
GROUP BY column1, column2
ORDER BY column1, column2

Example

Consider the CUSTOMERS table is having the following records −

SQL CODE

+----+----------+-----+-----------+----------+
| ID | NAME     | AGE | ADDRESS   | SALARY   |
+----+----------+-----+-----------+----------+
|  1 | Ramesh   |  32 | Ahmedabad |  2000.00 |
|  2 | Khilan   |  25 | Delhi     |  1500.00 |
|  3 | kaushik  |  23 | Kota      |  2000.00 |
|  4 | Chaitali |  25 | Mumbai    |  6500.00 |
|  5 | Hardik   |  27 | Bhopal    |  8500.00 |
|  6 | Komal    |  22 | MP        |  4500.00 |
|  7 | Muffy    |  24 | Indore    | 10000.00 |
+----+----------+-----+-----------+----------+

If you want to know the total amount of the salary on each customer, then the GROUP BY query would be as follows.

SQL CODE

SQL&gt; SELECT NAME, SUM(SALARY) FROM CUSTOMERS
   GROUP BY NAME;

This would produce the following result −

SQL CODE

+----------+-------------+
| NAME     | SUM(SALARY) |
+----------+-------------+
| Chaitali |     6500.00 |
| Hardik   |     8500.00 |
| kaushik  |     2000.00 |
| Khilan   |     1500.00 |
| Komal    |     4500.00 |
| Muffy    |    10000.00 |
| Ramesh   |     2000.00 |
+----------+-------------+

Now, let us look at a table where the CUSTOMERS table has the following records with duplicate names −

SQL CODE

+----+----------+-----+-----------+----------+
| ID | NAME     | AGE | ADDRESS   | SALARY   |
+----+----------+-----+-----------+----------+
|  1 | Ramesh   |  32 | Ahmedabad |  2000.00 |
|  2 | Ramesh   |  25 | Delhi     |  1500.00 |
|  3 | kaushik  |  23 | Kota      |  2000.00 |
|  4 | kaushik  |  25 | Mumbai    |  6500.00 |
|  5 | Hardik   |  27 | Bhopal    |  8500.00 |
|  6 | Komal    |  22 | MP        |  4500.00 |
|  7 | Muffy    |  24 | Indore    | 10000.00 |
+----+----------+-----+-----------+----------+

Now again, if you want to know the total amount of salary on each customer, then the GROUP BY query would be as follows −

SQL CODE

SQL&gt; SELECT NAME, SUM(SALARY) FROM CUSTOMERS
   GROUP BY NAME;

This would produce the following result −

SQL CODE

+---------+-------------+
| NAME    | SUM(SALARY) |
+---------+-------------+
| Hardik  |     8500.00 |
| kaushik |     8500.00 |
| Komal   |     4500.00 |
| Muffy   |    10000.00 |
| Ramesh  |     3500.00 |
+---------+-------------+

SQL - Distinct Keyword

The SQLDISTINCTkeyword is used in conjunction with the SELECT statement to eliminate all the duplicate records and fetching only unique records.

There may be a situation when you have multiple duplicate records in a table. While fetching such records, it makes more sense to fetch only those unique records instead of fetching duplicate records.

Syntax

The basic syntax of DISTINCT keyword to eliminate the duplicate records is as follows −

SQL CODE

SELECT DISTINCT column1, column2,.....columnN 
FROM table_name
WHERE [condition]

Example

Consider the CUSTOMERS table having the following records −

SQL CODE

+----+----------+-----+-----------+----------+
| ID | NAME     | AGE | ADDRESS   | SALARY   |
+----+----------+-----+-----------+----------+
|  1 | Ramesh   |  32 | Ahmedabad |  2000.00 |
|  2 | Khilan   |  25 | Delhi     |  1500.00 |
|  3 | kaushik  |  23 | Kota      |  2000.00 |
|  4 | Chaitali |  25 | Mumbai    |  6500.00 |
|  5 | Hardik   |  27 | Bhopal    |  8500.00 |
|  6 | Komal    |  22 | MP        |  4500.00 |
|  7 | Muffy    |  24 | Indore    | 10000.00 |
+----+----------+-----+-----------+----------+

First, let us see how the following SELECT query returns the duplicate salary records.

SQL CODE

SQL&gt; SELECT SALARY FROM CUSTOMERS
   ORDER BY SALARY;

This would produce the following result, where the salary (2000) is coming twice which is a duplicate record from the original table.

SQL CODE

+----------+
| SALARY   |
+----------+
|  1500.00 |
|  2000.00 |
|  2000.00 |
|  4500.00 |
|  6500.00 |
|  8500.00 |
| 10000.00 |
+----------+

Now, let us use the DISTINCT keyword with the above SELECT query and then see the result.

SQL CODE

SQL&gt; SELECT DISTINCT SALARY FROM CUSTOMERS
   ORDER BY SALARY;

This would produce the following result where we do not have any duplicate entry.

SQL CODE

+----------+
| SALARY   |
+----------+
|  1500.00 |
|  2000.00 |
|  4500.00 |
|  6500.00 |
|  8500.00 |
| 10000.00 |
+----------+

SQL - SORTING Results

The SQLORDER BYclause is used to sort the data in ascending or descending order, based on one or more columns. Some databases sort the query results in an ascending order by default.

Syntax

The basic syntax of the ORDER BY clause which would be used to sort the result in an ascending or descending order is as follows −

SQL CODE

SELECT column-list 
FROM table_name 
[WHERE condition] 
[ORDER BY column1, column2, .. columnN] [ASC | DESC];

You can use more than one column in the ORDER BY clause. Make sure that whatever column you are using to sort, that column should be in the column-list.

Example

Consider the CUSTOMERS table having the following records −

SQL CODE

+----+----------+-----+-----------+----------+
| ID | NAME     | AGE | ADDRESS   | SALARY   |
+----+----------+-----+-----------+----------+
|  1 | Ramesh   |  32 | Ahmedabad |  2000.00 |
|  2 | Khilan   |  25 | Delhi     |  1500.00 |
|  3 | kaushik  |  23 | Kota      |  2000.00 |
|  4 | Chaitali |  25 | Mumbai    |  6500.00 |
|  5 | Hardik   |  27 | Bhopal    |  8500.00 |
|  6 | Komal    |  22 | MP        |  4500.00 |
|  7 | Muffy    |  24 | Indore    | 10000.00 |
+----+----------+-----+-----------+----------+

Following is an example, which would sort the result in an ascending order by NAME and SALARY.

SQL CODE

SQL&gt; SELECT * FROM CUSTOMERS
   ORDER BY NAME, SALARY;

This would produce the following result −

SQL CODE

+----+----------+-----+-----------+----------+
| ID | NAME     | AGE | ADDRESS   | SALARY   |
+----+----------+-----+-----------+----------+
|  4 | Chaitali |  25 | Mumbai    |  6500.00 |
|  5 | Hardik   |  27 | Bhopal    |  8500.00 |
|  3 | kaushik  |  23 | Kota      |  2000.00 |
|  2 | Khilan   |  25 | Delhi     |  1500.00 |
|  6 | Komal    |  22 | MP        |  4500.00 |
|  7 | Muffy    |  24 | Indore    | 10000.00 |
|  1 | Ramesh   |  32 | Ahmedabad |  2000.00 |
+----+----------+-----+-----------+----------+

The following code block has an example, which would sort the result in a descending order by NAME.

SQL CODE

SQL&gt; SELECT * FROM CUSTOMERS
   ORDER BY NAME DESC;

This would produce the following result −

SQL CODE

+----+----------+-----+-----------+----------+
| ID | NAME     | AGE | ADDRESS   | SALARY   |
+----+----------+-----+-----------+----------+
|  1 | Ramesh   |  32 | Ahmedabad |  2000.00 |
|  7 | Muffy    |  24 | Indore    | 10000.00 |
|  6 | Komal    |  22 | MP        |  4500.00 |
|  2 | Khilan   |  25 | Delhi     |  1500.00 |
|  3 | kaushik  |  23 | Kota      |  2000.00 |
|  5 | Hardik   |  27 | Bhopal    |  8500.00 |
|  4 | Chaitali |  25 | Mumbai    |  6500.00 |
+----+----------+-----+-----------+----------+

To fetch the rows with their own preferred order, the SELECT query used would be as follows −

SQL CODE

SQL&gt; SELECT * FROM CUSTOMERS
   ORDER BY (CASE ADDRESS
   WHEN 'DELHI' 	 THEN 1
   WHEN 'BHOPAL' 	 THEN 2
   WHEN 'KOTA' 	 THEN 3
   WHEN 'AHMEDABAD' THEN 4
   WHEN 'MP' 	THEN 5
   ELSE 100 END) ASC, ADDRESS DESC;

This would produce the following result −

SQL CODE

+----+----------+-----+-----------+----------+
| ID | NAME     | AGE | ADDRESS   | SALARY   |
+----+----------+-----+-----------+----------+
|  2 | Khilan   |  25 | Delhi     |  1500.00 |
|  5 | Hardik   |  27 | Bhopal    |  8500.00 |
|  3 | kaushik  |  23 | Kota      |  2000.00 |
|  6 | Komal    |  22 | MP        |  4500.00 |
|  4 | Chaitali |  25 | Mumbai    |  6500.00 |
|  7 | Muffy    |  24 | Indore    | 10000.00 |
|  1 | Ramesh   |  32 | Ahmedabad |  2000.00 |
+----+----------+-----+-----------+----------+

This will sort the customers by ADDRESS in yourownoOrderof preference first and in a natural order for the remaining addresses. Also, the remaining Addresses will be sorted in the reverse alphabetical order.

SQL - Constraints

Constraints are the rules enforced on the data columns of a table. These are used to limit the type of data that can go into a table. This ensures the accuracy and reliability of the data in the database.

Constraints could be either on a column level or a table level. The column level constraints are applied only to one column, whereas the table level constraints are applied to the whole table.

Following are some of the most commonly used constraints available in SQL. These constraints have already been discussed inSQL - RDBMS Conceptschapter, but itâs worth to revise them at this point.

NOT NULL Constraint− Ensures that a column cannot have NULL value.
UNIQUE Constraint− Ensures that all values in a column are different.
PRIMARY Key− Uniquely identifies each row/record in a database table.
FOREIGN Key− Uniquely identifies a row/record in any of the given database table.
INDEX− Used to create and retrieve data from the database very quickly.

Constraints can be specified when a table is created with the CREATE TABLE statement or you can use the ALTER TABLE statement to create constraints even after the table is created.

Dropping Constraints

Any constraint that you have defined can be dropped using the ALTER TABLE command with the DROP CONSTRAINT option.

For example, to drop the primary key constraint in the EMPLOYEES table, you can use the following command.

SQL CODE

ALTER TABLE EMPLOYEES DROP CONSTRAINT EMPLOYEES_PK;

Some implementations may provide shortcuts for dropping certain constraints. For example, to drop the primary key constraint for a table in Oracle, you can use the following command.

SQL CODE

ALTER TABLE EMPLOYEES DROP PRIMARY KEY;

Some implementations allow you to disable constraints. Instead of permanently dropping a constraint from the database, you may want to temporarily disable the constraint and then enable it later.

Integrity Constraints

Integrity constraints are used to ensure accuracy and consistency of the data in a relational database. Data integrity is handled in a relational database through the concept of referential integrity.

There are many types of integrity constraints that play a role inReferential Integrity (RI). These constraints include Primary Key, Foreign Key, Unique Constraints and other constraints which are mentioned above.

SQL - Using Joins

The SQLJoinsclause is used to combine records from two or more tables in a database. A JOIN is a means for combining fields from two tables by using values common to each.

Consider the following two tables −

Table 1− CUSTOMERS Table

SQL CODE

+----+----------+-----+-----------+----------+
| ID | NAME     | AGE | ADDRESS   | SALARY   |
+----+----------+-----+-----------+----------+
|  1 | Ramesh   |  32 | Ahmedabad |  2000.00 |
|  2 | Khilan   |  25 | Delhi     |  1500.00 |
|  3 | kaushik  |  23 | Kota      |  2000.00 |
|  4 | Chaitali |  25 | Mumbai    |  6500.00 |
|  5 | Hardik   |  27 | Bhopal    |  8500.00 |
|  6 | Komal    |  22 | MP        |  4500.00 |
|  7 | Muffy    |  24 | Indore    | 10000.00 |
+----+----------+-----+-----------+----------+

Table 2− ORDERS Table

SQL CODE

+-----+---------------------+-------------+--------+
|OID  | DATE                | CUSTOMER_ID | AMOUNT |
+-----+---------------------+-------------+--------+
| 102 | 2009-10-08 00:00:00 |           3 |   3000 |
| 100 | 2009-10-08 00:00:00 |           3 |   1500 |
| 101 | 2009-11-20 00:00:00 |           2 |   1560 |
| 103 | 2008-05-20 00:00:00 |           4 |   2060 |
+-----+---------------------+-------------+--------+

Now, let us join these two tables in our SELECT statement as shown below.

SQL CODE

SQL&gt; SELECT ID, NAME, AGE, AMOUNT
   FROM CUSTOMERS, ORDERS
   WHERE  CUSTOMERS.ID = ORDERS.CUSTOMER_ID;

This would produce the following result.

SQL CODE

+----+----------+-----+--------+
| ID | NAME     | AGE | AMOUNT |
+----+----------+-----+--------+
|  3 | kaushik  |  23 |   3000 |
|  3 | kaushik  |  23 |   1500 |
|  2 | Khilan   |  25 |   1560 |
|  4 | Chaitali |  25 |   2060 |
+----+----------+-----+--------+

Here, it is noticeable that the join is performed in the WHERE clause. Several operators can be used to join tables, such as =, <, >, <>, <=, >=, !=, BETWEEN, LIKE, and NOT; they can all be used to join tables. However, the most common operator is the equal to symbol.

There are different types of joins available in SQL −

INNER JOIN− returns rows when there is a match in both tables.
LEFT JOIN− returns all rows from the left table, even if there are no matches in the right table.
RIGHT JOIN− returns all rows from the right table, even if there are no matches in the left table.
FULL JOIN− returns rows when there is a match in one of the tables.
SELF JOIN− is used to join a table to itself as if the table were two tables, temporarily renaming at least one table in the SQL statement.

Let us now discuss each of these joins in detail.

SQL - UNIONS CLAUSE

The SQL UNION clause/operator is used to combine the results of two or more SELECT statements without returning any duplicate rows.

To use this UNION clause, each SELECT statement must have

The same number of columns selected
The same number of column expressions
The same data type and
Have them in the same order

But they need not have to be in the same length.

Syntax

The basic syntax of aUNIONclause is as follows −

SQL CODE

SELECT column1 [, column2 ]
FROM table1 [, table2 ]
[WHERE condition]

UNION

SELECT column1 [, column2 ]
FROM table1 [, table2 ]
[WHERE condition]

Here, the given condition could be any given expression based on your requirement.

Example

Consider the following two tables.

Table 1− CUSTOMERS Table is as follows.

SQL CODE

+----+----------+-----+-----------+----------+
| ID | NAME     | AGE | ADDRESS   | SALARY   |
+----+----------+-----+-----------+----------+
|  1 | Ramesh   |  32 | Ahmedabad |  2000.00 |
|  2 | Khilan   |  25 | Delhi     |  1500.00 |
|  3 | kaushik  |  23 | Kota      |  2000.00 |
|  4 | Chaitali |  25 | Mumbai    |  6500.00 |
|  5 | Hardik   |  27 | Bhopal    |  8500.00 |
|  6 | Komal    |  22 | MP        |  4500.00 |
|  7 | Muffy    |  24 | Indore    | 10000.00 |
+----+----------+-----+-----------+----------+

Table 2− ORDERS Table is as follows.

SQL CODE

+-----+---------------------+-------------+--------+
|OID  | DATE                | CUSTOMER_ID | AMOUNT |
+-----+---------------------+-------------+--------+
| 102 | 2009-10-08 00:00:00 |           3 |   3000 |
| 100 | 2009-10-08 00:00:00 |           3 |   1500 |
| 101 | 2009-11-20 00:00:00 |           2 |   1560 |
| 103 | 2008-05-20 00:00:00 |           4 |   2060 |
+-----+---------------------+-------------+--------+

Now, let us join these two tables in our SELECT statement as follows −

SQL CODE

SQL&gt; SELECT  ID, NAME, AMOUNT, DATE
   FROM CUSTOMERS
   LEFT JOIN ORDERS
   ON CUSTOMERS.ID = ORDERS.CUSTOMER_ID
UNION
   SELECT  ID, NAME, AMOUNT, DATE
   FROM CUSTOMERS
   RIGHT JOIN ORDERS
   ON CUSTOMERS.ID = ORDERS.CUSTOMER_ID;

This would produce the following result −

SQL CODE

+------+----------+--------+---------------------+
| ID   | NAME     | AMOUNT | DATE                |
+------+----------+--------+---------------------+
|    1 | Ramesh   |   NULL | NULL                |
|    2 | Khilan   |   1560 | 2009-11-20 00:00:00 |
|    3 | kaushik  |   3000 | 2009-10-08 00:00:00 |
|    3 | kaushik  |   1500 | 2009-10-08 00:00:00 |
|    4 | Chaitali |   2060 | 2008-05-20 00:00:00 |
|    5 | Hardik   |   NULL | NULL                |
|    6 | Komal    |   NULL | NULL                |
|    7 | Muffy    |   NULL | NULL                |
+------+----------+--------+---------------------+

The UNION ALL Clause

The UNION ALL operator is used to combine the results of two SELECT statements including duplicate rows.

The same rules that apply to the UNION clause will apply to the UNION ALL operator.

Syntax

The basic syntax of theUNION ALLis as follows.

SQL CODE

SELECT column1 [, column2 ]
FROM table1 [, table2 ]
[WHERE condition]

UNION ALL

SELECT column1 [, column2 ]
FROM table1 [, table2 ]
[WHERE condition]

Here, the given condition could be any given expression based on your requirement.

Example

Consider the following two tables,

Table 1− CUSTOMERS Table is as follows.

SQL CODE

+----+----------+-----+-----------+----------+
| ID | NAME     | AGE | ADDRESS   | SALARY   |
+----+----------+-----+-----------+----------+
|  1 | Ramesh   |  32 | Ahmedabad |  2000.00 |
|  2 | Khilan   |  25 | Delhi     |  1500.00 |
|  3 | kaushik  |  23 | Kota      |  2000.00 |
|  4 | Chaitali |  25 | Mumbai    |  6500.00 |
|  5 | Hardik   |  27 | Bhopal    |  8500.00 |
|  6 | Komal    |  22 | MP        |  4500.00 |
|  7 | Muffy    |  24 | Indore    | 10000.00 |
+----+----------+-----+-----------+----------+

Table 2− ORDERS table is as follows.

SQL CODE

+-----+---------------------+-------------+--------+
|OID  | DATE                | CUSTOMER_ID | AMOUNT |
+-----+---------------------+-------------+--------+
| 102 | 2009-10-08 00:00:00 |           3 |   3000 |
| 100 | 2009-10-08 00:00:00 |           3 |   1500 |
| 101 | 2009-11-20 00:00:00 |           2 |   1560 |
| 103 | 2008-05-20 00:00:00 |           4 |   2060 |
+-----+---------------------+-------------+--------+

Now, let us join these two tables in our SELECT statement as follows −

SQL CODE

SQL&gt; SELECT  ID, NAME, AMOUNT, DATE
   FROM CUSTOMERS
   LEFT JOIN ORDERS
   ON CUSTOMERS.ID = ORDERS.CUSTOMER_ID
UNION ALL
   SELECT  ID, NAME, AMOUNT, DATE
   FROM CUSTOMERS
   RIGHT JOIN ORDERS
   ON CUSTOMERS.ID = ORDERS.CUSTOMER_ID;

This would produce the following result −

SQL CODE

+------+----------+--------+---------------------+
| ID   | NAME     | AMOUNT | DATE                |
+------+----------+--------+---------------------+
|    1 | Ramesh   |   NULL | NULL                |
|    2 | Khilan   |   1560 | 2009-11-20 00:00:00 |
|    3 | kaushik  |   3000 | 2009-10-08 00:00:00 |
|    3 | kaushik  |   1500 | 2009-10-08 00:00:00 |
|    4 | Chaitali |   2060 | 2008-05-20 00:00:00 |
|    5 | Hardik   |   NULL | NULL                |
|    6 | Komal    |   NULL | NULL                |
|    7 | Muffy    |   NULL | NULL                |
|    3 | kaushik  |   3000 | 2009-10-08 00:00:00 |
|    3 | kaushik  |   1500 | 2009-10-08 00:00:00 |
|    2 | Khilan   |   1560 | 2009-11-20 00:00:00 |
|    4 | Chaitali |   2060 | 2008-05-20 00:00:00 |
+------+----------+--------+---------------------+

There are two other clauses (i.e., operators), which are like the UNION clause.

SQLINTERSECT Clause− This is used to combine two SELECT statements, but returns rows only from the first SELECT statement that are identical to a row in the second SELECT statement.
SQLEXCEPT Clause− This combines two SELECT statements and returns rows from the first SELECT statement that are not returned by the second SELECT statement.

SQL - NULL Values

The SQLNULLis the term used to represent a missing value. A NULL value in a table is a value in a field that appears to be blank.

A field with a NULL value is a field with no value. It is very important to understand that a NULL value is different than a zero value or a field that contains spaces.

Syntax

The basic syntax ofNULLwhile creating a table.

SQL CODE

SQL&gt; CREATE TABLE CUSTOMERS(
   ID   INT              NOT NULL,
   NAME VARCHAR (20)     NOT NULL,
   AGE  INT              NOT NULL,
   ADDRESS  CHAR (25) ,
   SALARY   DECIMAL (18, 2),       
   PRIMARY KEY (ID)
);

Here,NOT NULLsignifies that column should always accept an explicit value of the given data type. There are two columns where we did not use NOT NULL, which means these columns could be NULL.

A field with a NULL value is the one that has been left blank during the record creation.

Example

The NULL value can cause problems when selecting data. However, because when comparing an unknown value to any other value, the result is always unknown and not included in the results. You must use theIS NULLorIS NOT NULLoperators to check for a NULL value.

Consider the following CUSTOMERS table having the records as shown below.

SQL CODE

+----+----------+-----+-----------+----------+
| ID | NAME     | AGE | ADDRESS   | SALARY   |
+----+----------+-----+-----------+----------+
|  1 | Ramesh   |  32 | Ahmedabad |  2000.00 |
|  2 | Khilan   |  25 | Delhi     |  1500.00 |
|  3 | kaushik  |  23 | Kota      |  2000.00 |
|  4 | Chaitali |  25 | Mumbai    |  6500.00 |
|  5 | Hardik   |  27 | Bhopal    |  8500.00 |
|  6 | Komal    |  22 | MP        |          |
|  7 | Muffy    |  24 | Indore    |          |
+----+----------+-----+-----------+----------+

Now, following is the usage of theIS NOT NULLoperator.

SQL CODE

SQL&gt; SELECT  ID, NAME, AGE, ADDRESS, SALARY
   FROM CUSTOMERS
   WHERE SALARY IS NOT NULL;

This would produce the following result −

SQL CODE

+----+----------+-----+-----------+----------+
| ID | NAME     | AGE | ADDRESS   | SALARY   |
+----+----------+-----+-----------+----------+
|  1 | Ramesh   |  32 | Ahmedabad |  2000.00 |
|  2 | Khilan   |  25 | Delhi     |  1500.00 |
|  3 | kaushik  |  23 | Kota      |  2000.00 |
|  4 | Chaitali |  25 | Mumbai    |  6500.00 |
|  5 | Hardik   |  27 | Bhopal    |  8500.00 |
+----+----------+-----+-----------+----------+

Now, following is the usage of theIS NULLoperator.

SQL CODE

SQL&gt; SELECT  ID, NAME, AGE, ADDRESS, SALARY
   FROM CUSTOMERS
   WHERE SALARY IS NULL;

This would produce the following result −

SQL CODE

+----+----------+-----+-----------+----------+
| ID | NAME     | AGE | ADDRESS   | SALARY   |
+----+----------+-----+-----------+----------+
|  6 | Komal    |  22 | MP        |          |
|  7 | Muffy    |  24 | Indore    |          |
+----+----------+-----+-----------+----------+

SQL - Alias Syntax

You can rename a table or a column temporarily by giving another name known asAlias. The use of table aliases is to rename a table in a specific SQL statement. The renaming is a temporary change and the actual table name does not change in the database. The column aliases are used to rename a table’s columns for the purpose of a particular SQL query.

Syntax

The basic syntax of atablealias is as follows.

SQL CODE

SELECT column1, column2....
FROM table_name AS alias_name
WHERE [condition];

The basic syntax of acolumnalias is as follows.

SQL CODE

SELECT column_name AS alias_name
FROM table_name
WHERE [condition];

Example

Consider the following two tables.

Table 1− CUSTOMERS Table is as follows.

SQL CODE

+----+----------+-----+-----------+----------+
| ID | NAME     | AGE | ADDRESS   | SALARY   |
+----+----------+-----+-----------+----------+
|  1 | Ramesh   |  32 | Ahmedabad |  2000.00 |
|  2 | Khilan   |  25 | Delhi     |  1500.00 |
|  3 | kaushik  |  23 | Kota      |  2000.00 |
|  4 | Chaitali |  25 | Mumbai    |  6500.00 |
|  5 | Hardik   |  27 | Bhopal    |  8500.00 |
|  6 | Komal    |  22 | MP        |  4500.00 |
|  7 | Muffy    |  24 | Indore    | 10000.00 |
+----+----------+-----+-----------+----------+

Table 2− ORDERS Table is as follows.

SQL CODE

+-----+---------------------+-------------+--------+
|OID  | DATE                | CUSTOMER_ID | AMOUNT |
+-----+---------------------+-------------+--------+
| 102 | 2009-10-08 00:00:00 |           3 |   3000 |
| 100 | 2009-10-08 00:00:00 |           3 |   1500 |
| 101 | 2009-11-20 00:00:00 |           2 |   1560 |
| 103 | 2008-05-20 00:00:00 |           4 |   2060 |
+-----+---------------------+-------------+--------+

Now, the following code block shows the usage of atable alias.

SQL CODE

SQL&gt; SELECT C.ID, C.NAME, C.AGE, O.AMOUNT 
   FROM CUSTOMERS AS C, ORDERS AS O
   WHERE  C.ID = O.CUSTOMER_ID;

This would produce the following result.

SQL CODE

+----+----------+-----+--------+
| ID | NAME     | AGE | AMOUNT |
+----+----------+-----+--------+
|  3 | kaushik  |  23 |   3000 |
|  3 | kaushik  |  23 |   1500 |
|  2 | Khilan   |  25 |   1560 |
|  4 | Chaitali |  25 |   2060 |
+----+----------+-----+--------+

Following is the usage of acolumn alias.

SQL CODE

SQL&gt; SELECT  ID AS CUSTOMER_ID, NAME AS CUSTOMER_NAME
   FROM CUSTOMERS
   WHERE SALARY IS NOT NULL;

This would produce the following result.

SQL CODE

+-------------+---------------+
| CUSTOMER_ID | CUSTOMER_NAME |
+-------------+---------------+
|           1 | Ramesh        |
|           2 | Khilan        |
|           3 | kaushik       |
|           4 | Chaitali      |
|           5 | Hardik        |
|           6 | Komal         |
|           7 | Muffy         |
+-------------+---------------+

SQL - Indexes

Indexes arespecial lookup tablesthat the database search engine can use to speed up data retrieval. Simply put, an index is a pointer to data in a table. An index in a database is very similar to an index in the back of a book.

For example, if you want to reference all pages in a book that discusses a certain topic, you first refer to the index, which lists all the topics alphabetically and are then referred to one or more specific page numbers.

An index helps to speed upSELECTqueries andWHEREclauses, but it slows down data input, with theUPDATEand theINSERTstatements. Indexes can be created or dropped with no effect on the data.

Creating an index involves theCREATE INDEXstatement, which allows you to name the index, to specify the table and which column or columns to index, and to indicate whether the index is in an ascending or descending order.

Indexes can also be unique, like theUNIQUEconstraint, in that the index prevents duplicate entries in the column or combination of columns on which there is an index.

The CREATE INDEX Command

The basic syntax of aCREATE INDEXis as follows.

SQL CODE

CREATE INDEX index_name ON table_name;

Single-Column Indexes

A single-column index is created based on only one table column. The basic syntax is as follows.

SQL CODE

CREATE INDEX index_name
ON table_name (column_name);

Unique Indexes

Unique indexes are used not only for performance, but also for data integrity. A unique index does not allow any duplicate values to be inserted into the table. The basic syntax is as follows.

SQL CODE

CREATE UNIQUE INDEX index_name
on table_name (column_name);

Composite Indexes

A composite index is an index on two or more columns of a table. Its basic syntax is as follows.

SQL CODE

CREATE INDEX index_name
on table_name (column1, column2);

Whether to create a single-column index or a composite index, take into consideration the column(s) that you may use very frequently in a query’s WHERE clause as filter conditions.

Should there be only one column used, a single-column index should be the choice. Should there be two or more columns that are frequently used in the WHERE clause as filters, the composite index would be the best choice.

Implicit Indexes

Implicit indexes are indexes that are automatically created by the database server when an object is created. Indexes are automatically created for primary key constraints and unique constraints.

The DROP INDEX Command

An index can be dropped using SQLDROPcommand. Care should be taken when dropping an index because the performance may either slow down or improve.

The basic syntax is as follows −

SQL CODE

DROP INDEX index_name;

You can check theINDEX Constraintchapter to see some actual examples on Indexes.

When should indexes be avoided?

Although indexes are intended to enhance a database’s performance, there are times when they should be avoided.

The following guidelines indicate when the use of an index should be reconsidered.

Indexes should not be used on small tables.
Tables that have frequent, large batch updates or insert operations.
Indexes should not be used on columns that contain a high number of NULL values.
Columns that are frequently manipulated should not be indexed.

SQL - ALTER TABLE Command

The SQLALTER TABLEcommand is used to add, delete or modify columns in an existing table. You should also use the ALTER TABLE command to add and drop various constraints on an existing table.

Syntax

The basic syntax of an ALTER TABLE command to add aNew Columnin an existing table is as follows.

SQL CODE

ALTER TABLE table_name ADD column_name datatype;

The basic syntax of an ALTER TABLE command toDROP COLUMNin an existing table is as follows.

SQL CODE

ALTER TABLE table_name DROP COLUMN column_name;

The basic syntax of an ALTER TABLE command to change theDATA TYPEof a column in a table is as follows.

SQL CODE

ALTER TABLE table_name MODIFY COLUMN column_name datatype;

The basic syntax of an ALTER TABLE command to add aNOT NULLconstraint to a column in a table is as follows.

SQL CODE

ALTER TABLE table_name MODIFY column_name datatype NOT NULL;

The basic syntax of ALTER TABLE toADD UNIQUE CONSTRAINTto a table is as follows.

SQL CODE

ALTER TABLE table_name 
ADD CONSTRAINT MyUniqueConstraint UNIQUE(column1, column2...);

The basic syntax of an ALTER TABLE command toADD CHECK CONSTRAINTto a table is as follows.

SQL CODE

ALTER TABLE table_name 
ADD CONSTRAINT MyUniqueConstraint CHECK (CONDITION);

The basic syntax of an ALTER TABLE command toADD PRIMARY KEYconstraint to a table is as follows.

SQL CODE

ALTER TABLE table_name 
ADD CONSTRAINT MyPrimaryKey PRIMARY KEY (column1, column2...);

The basic syntax of an ALTER TABLE command toDROP CONSTRAINTfrom a table is as follows.

SQL CODE

ALTER TABLE table_name 
DROP CONSTRAINT MyUniqueConstraint;

If you’re using MySQL, the code is as follows −

SQL CODE

ALTER TABLE table_name 
DROP INDEX MyUniqueConstraint;

The basic syntax of an ALTER TABLE command toDROP PRIMARY KEYconstraint from a table is as follows.

SQL CODE

ALTER TABLE table_name 
DROP CONSTRAINT MyPrimaryKey;

If you’re using MySQL, the code is as follows −

SQL CODE

ALTER TABLE table_name 
DROP PRIMARY KEY;

Example

Consider the CUSTOMERS table having the following records −

SQL CODE

+----+----------+-----+-----------+----------+
| ID | NAME     | AGE | ADDRESS   | SALARY   |
+----+----------+-----+-----------+----------+
|  1 | Ramesh   |  32 | Ahmedabad |  2000.00 |
|  2 | Khilan   |  25 | Delhi     |  1500.00 |
|  3 | kaushik  |  23 | Kota      |  2000.00 |
|  4 | Chaitali |  25 | Mumbai    |  6500.00 |
|  5 | Hardik   |  27 | Bhopal    |  8500.00 |
|  6 | Komal    |  22 | MP        |  4500.00 |
|  7 | Muffy    |  24 | Indore    | 10000.00 |
+----+----------+-----+-----------+----------+

Following is the example to ADD aNew Columnto an existing table −

SQL CODE

ALTER TABLE CUSTOMERS ADD SEX char(1);

Now, the CUSTOMERS table is changed and following would be output from the SELECT statement.

SQL CODE

+----+---------+-----+-----------+----------+------+
| ID | NAME    | AGE | ADDRESS   | SALARY   | SEX  |
+----+---------+-----+-----------+----------+------+
|  1 | Ramesh  |  32 | Ahmedabad |  2000.00 | NULL |
|  2 | Ramesh  |  25 | Delhi     |  1500.00 | NULL |
|  3 | kaushik |  23 | Kota      |  2000.00 | NULL |
|  4 | kaushik |  25 | Mumbai    |  6500.00 | NULL |
|  5 | Hardik  |  27 | Bhopal    |  8500.00 | NULL |
|  6 | Komal   |  22 | MP        |  4500.00 | NULL |
|  7 | Muffy   |  24 | Indore    | 10000.00 | NULL |
+----+---------+-----+-----------+----------+------+

Following is the example to DROP sex column from the existing table.

SQL CODE

ALTER TABLE CUSTOMERS DROP SEX;

Now, the CUSTOMERS table is changed and following would be the output from the SELECT statement.

SQL CODE

+----+---------+-----+-----------+----------+
| ID | NAME    | AGE | ADDRESS   | SALARY   |
+----+---------+-----+-----------+----------+
|  1 | Ramesh  |  32 | Ahmedabad |  2000.00 |
|  2 | Ramesh  |  25 | Delhi     |  1500.00 |
|  3 | kaushik |  23 | Kota      |  2000.00 |
|  4 | kaushik |  25 | Mumbai    |  6500.00 |
|  5 | Hardik  |  27 | Bhopal    |  8500.00 |
|  6 | Komal   |  22 | MP        |  4500.00 |
|  7 | Muffy   |  24 | Indore    | 10000.00 |
+----+---------+-----+-----------+----------+

SQL - TRUNCATE TABLE Command

The SQLTRUNCATE TABLEcommand is used to delete complete data from an existing table.

You can also use DROP TABLE command to delete complete table but it would remove complete table structure form the database and you would need to re-create this table once again if you wish you store some data.

Syntax

The basic syntax of aTRUNCATE TABLEcommand is as follows.

SQL CODE

TRUNCATE TABLE  table_name;

Example

Consider a CUSTOMERS table having the following records −

SQL CODE

+----+----------+-----+-----------+----------+
| ID | NAME     | AGE | ADDRESS   | SALARY   |
+----+----------+-----+-----------+----------+
|  1 | Ramesh   |  32 | Ahmedabad |  2000.00 |
|  2 | Khilan   |  25 | Delhi     |  1500.00 |
|  3 | kaushik  |  23 | Kota      |  2000.00 |
|  4 | Chaitali |  25 | Mumbai    |  6500.00 |
|  5 | Hardik   |  27 | Bhopal    |  8500.00 |
|  6 | Komal    |  22 | MP        |  4500.00 |
|  7 | Muffy    |  24 | Indore    | 10000.00 |
+----+----------+-----+-----------+----------+

Following is the example of a Truncate command.

SQL CODE

SQL &gt; TRUNCATE TABLE CUSTOMERS;

Now, the CUSTOMERS table is truncated and the output from SELECT statement will be as shown in the code block below −

SQL CODE

SQL&gt; SELECT * FROM CUSTOMERS;
Empty set (0.00 sec)

SQL - Using Views

A view is nothing more than a SQL statement that is stored in the database with an associated name. A view is actually a composition of a table in the form of a predefined SQL query.

A view can contain all rows of a table or select rows from a table. A view can be created from one or many tables which depends on the written SQL query to create a view.

Views, which are a type of virtual tables allow users to do the following −

Structure data in a way that users or classes of users find natural or intuitive.
Restrict access to the data in such a way that a user can see and (sometimes) modify exactly what they need and no more.
Summarize data from various tables which can be used to generate reports.

Creating Views

Database views are created using theCREATE VIEWstatement. Views can be created from a single table, multiple tables or another view.

To create a view, a user must have the appropriate system privilege according to the specific implementation.

The basicCREATE VIEWsyntax is as follows −

SQL CODE

CREATE VIEW view_name AS
SELECT column1, column2.....
FROM table_name
WHERE [condition];

You can include multiple tables in your SELECT statement in a similar way as you use them in a normal SQL SELECT query.

Example

Consider the CUSTOMERS table having the following records −

SQL CODE

+----+----------+-----+-----------+----------+
| ID | NAME     | AGE | ADDRESS   | SALARY   |
+----+----------+-----+-----------+----------+
|  1 | Ramesh   |  32 | Ahmedabad |  2000.00 |
|  2 | Khilan   |  25 | Delhi     |  1500.00 |
|  3 | kaushik  |  23 | Kota      |  2000.00 |
|  4 | Chaitali |  25 | Mumbai    |  6500.00 |
|  5 | Hardik   |  27 | Bhopal    |  8500.00 |
|  6 | Komal    |  22 | MP        |  4500.00 |
|  7 | Muffy    |  24 | Indore    | 10000.00 |
+----+----------+-----+-----------+----------+

Following is an example to create a view from the CUSTOMERS table. This view would be used to have customer name and age from the CUSTOMERS table.

SQL CODE

SQL &gt; CREATE VIEW CUSTOMERS_VIEW AS
SELECT name, age
FROM  CUSTOMERS;

Now, you can query CUSTOMERS_VIEW in a similar way as you query an actual table. Following is an example for the same.

SQL CODE

SQL &gt; SELECT * FROM CUSTOMERS_VIEW;

This would produce the following result.

SQL CODE

+----------+-----+
| name     | age |
+----------+-----+
| Ramesh   |  32 |
| Khilan   |  25 |
| kaushik  |  23 |
| Chaitali |  25 |
| Hardik   |  27 |
| Komal    |  22 |
| Muffy    |  24 |
+----------+-----+

The WITH CHECK OPTION

The WITH CHECK OPTION is a CREATE VIEW statement option. The purpose of the WITH CHECK OPTION is to ensure that all UPDATE and INSERTs satisfy the condition(s) in the view definition.

If they do not satisfy the condition(s), the UPDATE or INSERT returns an error.

The following code block has an example of creating same view CUSTOMERS_VIEW with the WITH CHECK OPTION.

SQL CODE

CREATE VIEW CUSTOMERS_VIEW AS
SELECT name, age
FROM  CUSTOMERS
WHERE age IS NOT NULL
WITH CHECK OPTION;

The WITH CHECK OPTION in this case should deny the entry of any NULL values in the view’s AGE column, because the view is defined by data that does not have a NULL value in the AGE column.

Updating a View

A view can be updated under certain conditions which are given below −

The SELECT clause may not contain the keyword DISTINCT.
The SELECT clause may not contain summary functions.
The SELECT clause may not contain set functions.
The SELECT clause may not contain set operators.
The SELECT clause may not contain an ORDER BY clause.
The FROM clause may not contain multiple tables.
The WHERE clause may not contain subqueries.
The query may not contain GROUP BY or HAVING.
Calculated columns may not be updated.
All NOT NULL columns from the base table must be included in the view in order for the INSERT query to function.

So, if a view satisfies all the above-mentioned rules then you can update that view. The following code block has an example to update the age of Ramesh.

SQL CODE

SQL &gt; UPDATE CUSTOMERS_VIEW
   SET AGE = 35
   WHERE name = 'Ramesh';

This would ultimately update the base table CUSTOMERS and the same would reflect in the view itself. Now, try to query the base table and the SELECT statement would produce the following result.

SQL CODE

+----+----------+-----+-----------+----------+
| ID | NAME     | AGE | ADDRESS   | SALARY   |
+----+----------+-----+-----------+----------+
|  1 | Ramesh   |  35 | Ahmedabad |  2000.00 |
|  2 | Khilan   |  25 | Delhi     |  1500.00 |
|  3 | kaushik  |  23 | Kota      |  2000.00 |
|  4 | Chaitali |  25 | Mumbai    |  6500.00 |
|  5 | Hardik   |  27 | Bhopal    |  8500.00 |
|  6 | Komal    |  22 | MP        |  4500.00 |
|  7 | Muffy    |  24 | Indore    | 10000.00 |
+----+----------+-----+-----------+----------+

Inserting Rows into a View

Rows of data can be inserted into a view. The same rules that apply to the UPDATE command also apply to the INSERT command.

Here, we cannot insert rows in the CUSTOMERS_VIEW because we have not included all the NOT NULL columns in this view, otherwise you can insert rows in a view in a similar way as you insert them in a table.

Deleting Rows into a View

Rows of data can be deleted from a view. The same rules that apply to the UPDATE and INSERT commands apply to the DELETE command.

Following is an example to delete a record having AGE = 22.

SQL CODE

SQL &gt; DELETE FROM CUSTOMERS_VIEW
   WHERE age = 22;

This would ultimately delete a row from the base table CUSTOMERS and the same would reflect in the view itself. Now, try to query the base table and the SELECT statement would produce the following result.

SQL CODE

+----+----------+-----+-----------+----------+
| ID | NAME     | AGE | ADDRESS   | SALARY   |
+----+----------+-----+-----------+----------+
|  1 | Ramesh   |  35 | Ahmedabad |  2000.00 |
|  2 | Khilan   |  25 | Delhi     |  1500.00 |
|  3 | kaushik  |  23 | Kota      |  2000.00 |
|  4 | Chaitali |  25 | Mumbai    |  6500.00 |
|  5 | Hardik   |  27 | Bhopal    |  8500.00 |
|  7 | Muffy    |  24 | Indore    | 10000.00 |
+----+----------+-----+-----------+----------+

Dropping Views

Obviously, where you have a view, you need a way to drop the view if it is no longer needed. The syntax is very simple and is given below −

SQL CODE

DROP VIEW view_name;

Following is an example to drop the CUSTOMERS_VIEW from the CUSTOMERS table.

SQL CODE

DROP VIEW CUSTOMERS_VIEW;

SQL - Having Clause

TheHAVING Clauseenables you to specify conditions that filter which group results appear in the results.

The WHERE clause places conditions on the selected columns, whereas the HAVING clause places conditions on groups created by the GROUP BY clause.

Syntax

The following code block shows the position of the HAVING Clause in a query.

SQL CODE

SELECT
FROM
WHERE
GROUP BY
HAVING
ORDER BY

The HAVING clause must follow the GROUP BY clause in a query and must also precede the ORDER BY clause if used. The following code block has the syntax of the SELECT statement including the HAVING clause −

SQL CODE

SELECT column1, column2
FROM table1, table2
WHERE [ conditions ]
GROUP BY column1, column2
HAVING [ conditions ]
ORDER BY column1, column2

Example

Consider the CUSTOMERS table having the following records.

SQL CODE

+----+----------+-----+-----------+----------+
| ID | NAME     | AGE | ADDRESS   | SALARY   |
+----+----------+-----+-----------+----------+
|  1 | Ramesh   |  32 | Ahmedabad |  2000.00 |
|  2 | Khilan   |  25 | Delhi     |  1500.00 |
|  3 | kaushik  |  23 | Kota      |  2000.00 |
|  4 | Chaitali |  25 | Mumbai    |  6500.00 |
|  5 | Hardik   |  27 | Bhopal    |  8500.00 |
|  6 | Komal    |  22 | MP        |  4500.00 |
|  7 | Muffy    |  24 | Indore    | 10000.00 |
+----+----------+-----+-----------+----------+

Following is an example, which would display a record for a similar age count that would be more than or equal to 2.

SQL CODE

SQL &gt; SELECT ID, NAME, AGE, ADDRESS, SALARY
FROM CUSTOMERS
GROUP BY age
HAVING COUNT(age) &gt;= 2;

This would produce the following result −

SQL CODE

+----+--------+-----+---------+---------+
| ID | NAME   | AGE | ADDRESS | SALARY  |
+----+--------+-----+---------+---------+
|  2 | Khilan |  25 | Delhi   | 1500.00 |
+----+--------+-----+---------+---------+

SQL - Transactions

A transaction is a unit of work that is performed against a database. Transactions are units or sequences of work accomplished in a logical order, whether in a manual fashion by a user or automatically by some sort of a database program.

A transaction is the propagation of one or more changes to the database. For example, if you are creating a record or updating a record or deleting a record from the table, then you are performing a transaction on that table. It is important to control these transactions to ensure the data integrity and to handle database errors.

Practically, you will club many SQL queries into a group and you will execute all of them together as a part of a transaction.

Properties of Transactions

Transactions have the following four standard properties, usually referred to by the acronymACID.

Atomicity− ensures that all operations within the work unit are completed successfully. Otherwise, the transaction is aborted at the point of failure and all the previous operations are rolled back to their former state.
Consistency− ensures that the database properly changes states upon a successfully committed transaction.
Isolation− enables transactions to operate independently of and transparent to each other.
Durability− ensures that the result or effect of a committed transaction persists in case of a system failure.

Transaction Control

The following commands are used to control transactions.

COMMIT− to save the changes.
ROLLBACK− to roll back the changes.
SAVEPOINT− creates points within the groups of transactions in which to ROLLBACK.
SET TRANSACTION− Places a name on a transaction.

Transactional Control Commands

Transactional control commands are only used with theDML Commandssuch as - INSERT, UPDATE and DELETE only. They cannot be used while creating tables or dropping them because these operations are automatically committed in the database.

The COMMIT Command

The COMMIT command is the transactional command used to save changes invoked by a transaction to the database.

The COMMIT command is the transactional command used to save changes invoked by a transaction to the database. The COMMIT command saves all the transactions to the database since the last COMMIT or ROLLBACK command.

The syntax for the COMMIT command is as follows.

SQL CODE

COMMIT;

Example

Consider the CUSTOMERS table having the following records −

SQL CODE

+----+----------+-----+-----------+----------+
| ID | NAME     | AGE | ADDRESS   | SALARY   |
+----+----------+-----+-----------+----------+
|  1 | Ramesh   |  32 | Ahmedabad |  2000.00 |
|  2 | Khilan   |  25 | Delhi     |  1500.00 |
|  3 | kaushik  |  23 | Kota      |  2000.00 |
|  4 | Chaitali |  25 | Mumbai    |  6500.00 |
|  5 | Hardik   |  27 | Bhopal    |  8500.00 |
|  6 | Komal    |  22 | MP        |  4500.00 |
|  7 | Muffy    |  24 | Indore    | 10000.00 |
+----+----------+-----+-----------+----------+

Following is an example which would delete those records from the table which have age = 25 and then COMMIT the changes in the database.

SQL CODE

SQL&gt; DELETE FROM CUSTOMERS
   WHERE AGE = 25;
SQL&gt; COMMIT;

Thus, two rows from the table would be deleted and the SELECT statement would produce the following result.

SQL CODE

+----+----------+-----+-----------+----------+
| ID | NAME     | AGE | ADDRESS   | SALARY   |
+----+----------+-----+-----------+----------+
|  1 | Ramesh   |  32 | Ahmedabad |  2000.00 |
|  3 | kaushik  |  23 | Kota      |  2000.00 |
|  5 | Hardik   |  27 | Bhopal    |  8500.00 |
|  6 | Komal    |  22 | MP        |  4500.00 |
|  7 | Muffy    |  24 | Indore    | 10000.00 |
+----+----------+-----+-----------+----------+

The ROLLBACK Command

The ROLLBACK command is the transactional command used to undo transactions that have not already been saved to the database. This command can only be used to undo transactions since the last COMMIT or ROLLBACK command was issued.

The syntax for a ROLLBACK command is as follows −

SQL CODE

ROLLBACK;

Example

Consider the CUSTOMERS table having the following records −

SQL CODE

+----+----------+-----+-----------+----------+
| ID | NAME     | AGE | ADDRESS   | SALARY   |
+----+----------+-----+-----------+----------+
|  1 | Ramesh   |  32 | Ahmedabad |  2000.00 |
|  2 | Khilan   |  25 | Delhi     |  1500.00 |
|  3 | kaushik  |  23 | Kota      |  2000.00 |
|  4 | Chaitali |  25 | Mumbai    |  6500.00 |
|  5 | Hardik   |  27 | Bhopal    |  8500.00 |
|  6 | Komal    |  22 | MP        |  4500.00 |
|  7 | Muffy    |  24 | Indore    | 10000.00 |
+----+----------+-----+-----------+----------+

Following is an example, which would delete those records from the table which have the age = 25 and then ROLLBACK the changes in the database.

SQL CODE

SQL&gt; DELETE FROM CUSTOMERS
   WHERE AGE = 25;
SQL&gt; ROLLBACK;

Thus, the delete operation would not impact the table and the SELECT statement would produce the following result.

SQL CODE

+----+----------+-----+-----------+----------+
| ID | NAME     | AGE | ADDRESS   | SALARY   |
+----+----------+-----+-----------+----------+
|  1 | Ramesh   |  32 | Ahmedabad |  2000.00 |
|  2 | Khilan   |  25 | Delhi     |  1500.00 |
|  3 | kaushik  |  23 | Kota      |  2000.00 |
|  4 | Chaitali |  25 | Mumbai    |  6500.00 |
|  5 | Hardik   |  27 | Bhopal    |  8500.00 |
|  6 | Komal    |  22 | MP        |  4500.00 |
|  7 | Muffy    |  24 | Indore    | 10000.00 |
+----+----------+-----+-----------+----------+

The SAVEPOINT Command

A SAVEPOINT is a point in a transaction when you can roll the transaction back to a certain point without rolling back the entire transaction.

The syntax for a SAVEPOINT command is as shown below.

SQL CODE

SAVEPOINT SAVEPOINT_NAME;

This command serves only in the creation of a SAVEPOINT among all the transactional statements. The ROLLBACK command is used to undo a group of transactions.

The syntax for rolling back to a SAVEPOINT is as shown below.

SQL CODE

ROLLBACK TO SAVEPOINT_NAME;

Following is an example where you plan to delete the three different records from the CUSTOMERS table. You want to create a SAVEPOINT before each delete, so that you can ROLLBACK to any SAVEPOINT at any time to return the appropriate data to its original state.

Example

Consider the CUSTOMERS table having the following records.

SQL CODE

+----+----------+-----+-----------+----------+
| ID | NAME     | AGE | ADDRESS   | SALARY   |
+----+----------+-----+-----------+----------+
|  1 | Ramesh   |  32 | Ahmedabad |  2000.00 |
|  2 | Khilan   |  25 | Delhi     |  1500.00 |
|  3 | kaushik  |  23 | Kota      |  2000.00 |
|  4 | Chaitali |  25 | Mumbai    |  6500.00 |
|  5 | Hardik   |  27 | Bhopal    |  8500.00 |
|  6 | Komal    |  22 | MP        |  4500.00 |
|  7 | Muffy    |  24 | Indore    | 10000.00 |
+----+----------+-----+-----------+----------+

The following code block contains the series of operations.

SQL CODE

SQL&gt; SAVEPOINT SP1;
Savepoint created.
SQL&gt; DELETE FROM CUSTOMERS WHERE ID=1;
1 row deleted.
SQL&gt; SAVEPOINT SP2;
Savepoint created.
SQL&gt; DELETE FROM CUSTOMERS WHERE ID=2;
1 row deleted.
SQL&gt; SAVEPOINT SP3;
Savepoint created.
SQL&gt; DELETE FROM CUSTOMERS WHERE ID=3;
1 row deleted.

Now that the three deletions have taken place, let us assume that you have changed your mind and decided to ROLLBACK to the SAVEPOINT that you identified as SP2. Because SP2 was created after the first deletion, the last two deletions are undone −

SQL CODE

SQL&gt; ROLLBACK TO SP2;
Rollback complete.

Notice that only the first deletion took place since you rolled back to SP2.

SQL CODE

SQL&gt; SELECT * FROM CUSTOMERS;
+----+----------+-----+-----------+----------+
| ID | NAME     | AGE | ADDRESS   | SALARY   |
+----+----------+-----+-----------+----------+
|  2 | Khilan   |  25 | Delhi     |  1500.00 |
|  3 | kaushik  |  23 | Kota      |  2000.00 |
|  4 | Chaitali |  25 | Mumbai    |  6500.00 |
|  5 | Hardik   |  27 | Bhopal    |  8500.00 |
|  6 | Komal    |  22 | MP        |  4500.00 |
|  7 | Muffy    |  24 | Indore    | 10000.00 |
+----+----------+-----+-----------+----------+
6 rows selected.

The RELEASE SAVEPOINT Command

The RELEASE SAVEPOINT command is used to remove a SAVEPOINT that you have created.

The syntax for a RELEASE SAVEPOINT command is as follows.

SQL CODE

RELEASE SAVEPOINT SAVEPOINT_NAME;

Once a SAVEPOINT has been released, you can no longer use the ROLLBACK command to undo transactions performed since the last SAVEPOINT.

The SET TRANSACTION Command

The SET TRANSACTION command can be used to initiate a database transaction. This command is used to specify characteristics for the transaction that follows. For example, you can specify a transaction to be read only or read write.

The syntax for a SET TRANSACTION command is as follows.

SQL CODE

SET TRANSACTION [ READ WRITE | READ ONLY ];

SQL - Wildcard Operators

We have already discussed about the SQL LIKE operator, which is used to compare a value to similar values using the wildcard operators.

SQL supports two wildcard operators in conjunction with the LIKE operator which are explained in detail in the following table.

Sr.No.Wildcard & Description1The percent sign (%)Matches one or more characters.Note− MS Access uses the asterisk (*) wildcard character instead of the percent sign (%) wildcard character.2The underscore ()Matches one character.Note− MS Access uses a question mark (?) instead of the underscore () to match any one character.

The percent sign represents zero, one or multiple characters. The underscore represents a single number or a character. These symbols can be used in combinations.

Syntax

The basic syntax of a ’%’ and a ’_’ operator is as follows.

SQL CODE

SELECT * FROM table_name
WHERE column LIKE 'XXXX%'

or 

SELECT * FROM table_name
WHERE column LIKE '%XXXX%'

or

SELECT * FROM table_name
WHERE column LIKE 'XXXX_'

or

SELECT * FROM table_name
WHERE column LIKE '_XXXX'

or

SELECT * FROM table_name
WHERE column LIKE '_XXXX_'

You can combine N number of conditions using the AND or the OR operators. Here, XXXX could be any numeric or string value.

Example

The following table has a number of examples showing the WHERE part having different LIKE clauses with ’%’ and ’_’ operators.

Let us take a real example, consider the CUSTOMERS table having the following records.

SQL CODE

+----+----------+-----+-----------+----------+
| ID | NAME     | AGE | ADDRESS   | SALARY   |
+----+----------+-----+-----------+----------+
|  1 | Ramesh   |  32 | Ahmedabad |  2000.00 |
|  2 | Khilan   |  25 | Delhi     |  1500.00 |
|  3 | kaushik  |  23 | Kota      |  2000.00 |
|  4 | Chaitali |  25 | Mumbai    |  6500.00 |
|  5 | Hardik   |  27 | Bhopal    |  8500.00 |
|  6 | Komal    |  22 | MP        |  4500.00 |
|  7 | Muffy    |  24 | Indore    | 10000.00 |
+----+----------+-----+-----------+----------+

The following code block is an example, which would display all the records from the CUSTOMERS table where the SALARY starts with 200.

SQL CODE

SQL&gt; SELECT * FROM CUSTOMERS
WHERE SALARY LIKE '200%';

This would produce the following result.

SQL CODE

+----+----------+-----+-----------+----------+
| ID | NAME     | AGE | ADDRESS   | SALARY   |
+----+----------+-----+-----------+----------+
|  1 | Ramesh   |  32 | Ahmedabad |  2000.00 |
|  3 | kaushik  |  23 | Kota      |  2000.00 |
+----+----------+-----+-----------+----------+

SQL - Date Functions

The following table has a list of all the important Date and Time related functions available through SQL. There are various other functions supported by your RDBMS. The given list is based on MySQL RDBMS.

Sr.No.Function & Description1ADDDATE()Adds dates2ADDTIME()Adds time3CONVERT_TZ()Converts from one timezone to another4CURDATE()Returns the current date5CURRENT_DATE(), CURRENT_DATESynonyms for CURDATE()6CURRENT_TIME(), CURRENT_TIMESynonyms for CURTIME()7CURRENT_TIMESTAMP(), CURRENT_TIMESTAMPSynonyms for NOW()8CURTIME()Returns the current time9DATE_ADD()Adds two dates10DATE_FORMAT()Formats date as specified11DATE_SUB()Subtracts two dates12DATE()Extracts the date part of a date or datetime expression13DATEDIFF()Subtracts two dates14DAY()Synonym for DAYOFMONTH()15DAYNAME()Returns the name of the weekday16DAYOFMONTH()Returns the day of the month (1-31)17DAYOFWEEK()Returns the weekday index of the argument18DAYOFYEAR()Returns the day of the year (1-366)19EXTRACTExtracts part of a date20FROM_DAYS()Converts a day number to a date21FROM_UNIXTIME()Formats date as a UNIX timestamp22HOUR()Extracts the hour23LAST_DAYReturns the last day of the month for the argument24LOCALTIME(), LOCALTIMESynonym for NOW()25LOCALTIMESTAMP, LOCALTIMESTAMP()Synonym for NOW()26MAKEDATE()Creates a date from the year and day of year27MAKETIMEMAKETIME()28MICROSECOND()Returns the microseconds from argument29MINUTE()Returns the minute from the argument30MONTH()Return the month from the date passed31MONTHNAME()Returns the name of the month32NOW()Returns the current date and time33PERIOD_ADD()Adds a period to a year-month34PERIOD_DIFF()Returns the number of months between periods35QUARTER()Returns the quarter from a date argument36SEC_TO_TIME()Converts seconds to ‘HH:MM:SS’ format37SECOND()Returns the second (0-59)38STR_TO_DATE()Converts a string to a date39SUBDATE()When invoked with three arguments a synonym for DATE_SUB()40SUBTIME()Subtracts times41SYSDATE()Returns the time at which the function executes42TIME_FORMAT()Formats as time43TIME_TO_SEC()Returns the argument converted to seconds44TIME()Extracts the time portion of the expression passed45TIMEDIFF()Subtracts time46TIMESTAMP()With a single argument this function returns the date or datetime expression. With two arguments, the sum of the arguments47TIMESTAMPADD()Adds an interval to a datetime expression48TIMESTAMPDIFF()Subtracts an interval from a datetime expression49TO_DAYS()Returns the date argument converted to days50UNIX_TIMESTAMP()Returns a UNIX timestamp51UTC_DATE()Returns the current UTC date52UTC_TIME()Returns the current UTC time53UTC_TIMESTAMP()Returns the current UTC date and time54WEEK()Returns the week number55WEEKDAY()Returns the weekday index56WEEKOFYEAR()Returns the calendar week of the date (1-53)57YEAR()Returns the year58YEARWEEK()Returns the year and week

ADDDATE(date,INTERVAL expr unit), ADDDATE(expr,days)

When invoked with the INTERVAL form of the second argument, ADDDATE() is a synonym for DATE_ADD(). The related function SUBDATE() is a synonym for DATE_SUB(). For information on the INTERVAL unit argument, see the discussion for DATE_ADD().

SQL CODE

mysql&gt; SELECT DATE_ADD('1998-01-02', INTERVAL 31 DAY);
+---------------------------------------------------------+
| DATE_ADD('1998-01-02', INTERVAL 31 DAY)                 |
+---------------------------------------------------------+
| 1998-02-02                                              |
+---------------------------------------------------------+
1 row in set (0.00 sec)

mysql&gt; SELECT ADDDATE('1998-01-02', INTERVAL 31 DAY);
+---------------------------------------------------------+
| ADDDATE('1998-01-02', INTERVAL 31 DAY)                  |
+---------------------------------------------------------+
| 1998-02-02                                              |
+---------------------------------------------------------+
1 row in set (0.00 sec)

When invoked with the days form of the second argument, MySQL treats it as an integer number of days to be added to expr.

SQL CODE

mysql&gt; SELECT ADDDATE('1998-01-02', 31);
+---------------------------------------------------------+
| DATE_ADD('1998-01-02', INTERVAL 31 DAY)                 |
+---------------------------------------------------------+
| 1998-02-02                                              |
+---------------------------------------------------------+
1 row in set (0.00 sec)

ADDTIME(expr1,expr2)

ADDTIME() adds expr2 to expr1 and returns the result. The expr1 is a time or datetime expression, while the expr2 is a time expression.

SQL CODE

mysql&gt; SELECT ADDTIME('1997-12-31 23:59:59.999999','1 1:1:1.000002');
+---------------------------------------------------------+
| DATE_ADD('1997-12-31 23:59:59.999999','1 1:1:1.000002') |
+---------------------------------------------------------+
| 1998-01-02 01:01:01.000001                              |
+---------------------------------------------------------+
1 row in set (0.00 sec)

CONVERT_TZ(dt,from_tz,to_tz)

This converts a datetime value dt from the time zone given by from_tz to the time zone given by to_tz and returns the resulting value. This function returns NULL if the arguments are invalid.

SQL CODE

mysql&gt; SELECT CONVERT_TZ('2004-01-01 12:00:00','GMT','MET');
+---------------------------------------------------------+
| CONVERT_TZ('2004-01-01 12:00:00','GMT','MET')           |
+---------------------------------------------------------+
| 2004-01-01 13:00:00                                     |
+---------------------------------------------------------+
1 row in set (0.00 sec)

mysql&gt; SELECT CONVERT_TZ('2004-01-01 12:00:00','+00:00','+10:00');
+---------------------------------------------------------+
| CONVERT_TZ('2004-01-01 12:00:00','+00:00','+10:00')     |
+---------------------------------------------------------+
| 2004-01-01 22:00:00                                     |
+---------------------------------------------------------+
1 row in set (0.00 sec)

CURDATE()

Returns the current date as a value in ‘YYYY-MM-DD’ or YYYYMMDD format, depending on whether the function is used in a string or in a numeric context.

SQL CODE

mysql&gt; SELECT CURDATE();
+---------------------------------------------------------+
| CURDATE()                                               |
+---------------------------------------------------------+
| 1997-12-15                                              |
+---------------------------------------------------------+
1 row in set (0.00 sec)

mysql&gt; SELECT CURDATE() + 0;
+---------------------------------------------------------+
| CURDATE() + 0                                           |
+---------------------------------------------------------+
| 19971215                                                |
+---------------------------------------------------------+
1 row in set (0.00 sec)

CURRENT_DATE and CURRENT_DATE()

CURRENT_DATE and CURRENT_DATE() are synonyms for CURDATE()

CURTIME()

Returns the current time as a value in ‘HH:MM:SS’ or HHMMSS format, depending on whether the function is used in a string or in a numeric context. The value is expressed in the current time zone.

SQL CODE

mysql&gt; SELECT CURTIME();
+---------------------------------------------------------+
| CURTIME()                                               |
+---------------------------------------------------------+
| 23:50:26                                                |
+---------------------------------------------------------+
1 row in set (0.00 sec)

mysql&gt; SELECT CURTIME() + 0;
+---------------------------------------------------------+
| CURTIME() + 0                                           |
+---------------------------------------------------------+
| 235026                                                  |
+---------------------------------------------------------+
1 row in set (0.00 sec)

CURRENT_TIME and CURRENT_TIME()

CURRENT_TIME and CURRENT_TIME() are synonyms for CURTIME().

CURRENT_TIMESTAMP and CURRENT_TIMESTAMP()

CURRENT_TIMESTAMP and CURRENT_TIMESTAMP() are synonyms for NOW().

DATE(expr)

Extracts the date part of the date or datetime expression expr.

SQL CODE

mysql&gt; SELECT DATE('2003-12-31 01:02:03');
+---------------------------------------------------------+
| DATE('2003-12-31 01:02:03')                             |
+---------------------------------------------------------+
|  2003-12-31                                             |
+---------------------------------------------------------+
1 row in set (0.00 sec)

DATEDIFF(expr1,expr2)

DATEDIFF() returns expr1 . expr2 expressed as a value in days from one date to the other. Both expr1 and expr2 are date or date-and-time expressions. Only the date parts of the values are used in the calculation.

SQL CODE

mysql&gt; SELECT DATEDIFF('1997-12-31 23:59:59','1997-12-30');
+---------------------------------------------------------+
| DATEDIFF('1997-12-31 23:59:59','1997-12-30')            |
+---------------------------------------------------------+
| 1                                                       |
+---------------------------------------------------------+
1 row in set (0.00 sec)

DATE_ADD(date,INTERVAL expr unit), DATE_SUB(date,INTERVAL expr unit)

These functions perform date arithmetic. Thedateis a DATETIME or DATE value specifying the starting date. Theexpris an expression specifying the interval value to be added or subtracted from the starting date. The expr is a string; it may start with a ’-’ for negative intervals.

Aunitis a keyword indicating the units in which the expression should be interpreted.

TheINTERVALkeyword and the unit specifier are not case sensitive.

The following table shows the expected form of the expr argument for each unit value.

unit ValueExpected exprFormatMICROSECONDMICROSECONDSSECONDSECONDSMINUTEMINUTESHOURHOURSDAYDAYSWEEKWEEKSMONTHMONTHSQUARTERQUARTERSYEARYEARSSECOND_MICROSECOND’SECONDS.MICROSECONDS’MINUTE_MICROSECOND’MINUTES.MICROSECONDS’MINUTE_SECOND’MINUTES:SECONDS’HOUR_MICROSECOND’HOURS.MICROSECONDS’HOUR_SECOND’HOURS:MINUTES:SECONDS’HOUR_MINUTE’HOURS:MINUTES’DAY_MICROSECOND’DAYS.MICROSECONDS’DAY_SECOND’DAYS HOURS:MINUTES:SECONDS’DAY_MINUTE’DAYS HOURS:MINUTES’DAY_HOUR’DAYS HOURS’YEAR_MONTH’YEARS-MONTHS’

The valuesQUARTERandWEEKare available from the MySQL 5.0.0. version.

SQL CODE

mysql&gt; SELECT DATE_ADD('1997-12-31 23:59:59', 
   -&gt; INTERVAL '1:1' MINUTE_SECOND);
+---------------------------------------------------------+
| DATE_ADD('1997-12-31 23:59:59', INTERVAL...             |
+---------------------------------------------------------+
| 1998-01-01 00:01:00                                     |
+---------------------------------------------------------+
1 row in set (0.00 sec)

mysql&gt; SELECT DATE_ADD('1999-01-01', INTERVAL 1 HOUR);
+---------------------------------------------------------+
| DATE_ADD('1999-01-01', INTERVAL 1 HOUR)                 |
+---------------------------------------------------------+
| 1999-01-01 01:00:00                                     |
+---------------------------------------------------------+
1 row in set (0.00 sec)

DATE_FORMAT(date,format)

This command formats the date value as per the format string.

The following specifiers may be used in the format string. The ’%’ character is required before the format specifier characters.

Sr.No.Specifier & Description1%aAbbreviated weekday name (Sun..Sat)2%bAbbreviated month name (Jan..Dec)3%cMonth, numeric (0..12)4%DDay of the month with English suffix (0th, 1st, 2nd, 3rd, .)5%dDay of the month, numeric (00..31)6%eDay of the month, numeric (0..31)7%fMicroseconds (000000..999999)8%HHour (00..23)9%hHour (01..12)10%IHour (01..12)11%iMinutes, numeric (00..59)12%jDay of year (001..366)13%kHour (0..23)14%lHour (1..12)15%MMonth name (January..December)16%mMonth, numeric (00..12)17%pAM or PM18%rTime, 12-hour (hh:mm:ss followed by AM or PM)19%SSeconds (00..59)20%sSeconds (00..59)21%TTime, 24-hour (hh:mm:ss)22%UWeek (00..53), where Sunday is the first day of the week23%uWeek (00..53), where Monday is the first day of the week24%VWeek (01..53), where Sunday is the first day of the week; used with %X25%vWeek (01..53), where Monday is the first day of the week; used with %x26%WWeekday name (Sunday..Saturday)27%wDay of the week (0= Sunday..6= Saturday)28%XYear for the week where Sunday is the first day of the week, numeric, four digits; used with %V29%xYear for the week, where Monday is the first day of the week, numeric, four digits; used with %v30%YYear, numeric, four digits31%yYear, numeric (two digits)32%%A literal .%. character33%xx, for any.x. not listed above

SQL CODE

mysql&gt; SELECT DATE_FORMAT('1997-10-04 22:23:00', '%W %M %Y');
+---------------------------------------------------------+
| DATE_FORMAT('1997-10-04 22:23:00', '%W %M %Y')          |
+---------------------------------------------------------+
| Saturday October 1997                                   |
+---------------------------------------------------------+
1 row in set (0.00 sec)

mysql&gt; SELECT DATE_FORMAT('1997-10-04 22:23:00'
   -&gt; '%H %k %I %r %T %S %w');
+---------------------------------------------------------+
| DATE_FORMAT('1997-10-04 22:23:00.......                 |
+---------------------------------------------------------+
|  22 22 10 10:23:00 PM 22:23:00 00 6                     |
+---------------------------------------------------------+
1 row in set (0.00 sec)

DATE_SUB(date,INTERVAL expr unit)

This is similar to the DATE_ADD() function.

DAY(date)

The DAY() is a synonym for the DAYOFMONTH() function.

DAYNAME(date)

Returns the name of the weekday for date.

SQL CODE

mysql&gt; SELECT DAYNAME('1998-02-05');
+---------------------------------------------------------+
| DAYNAME('1998-02-05')                                   |
+---------------------------------------------------------+
| Thursday                                                |
+---------------------------------------------------------+
1 row in set (0.00 sec)

DAYOFMONTH(date)

Returns the day of the month for date, in the range 0 to 31.

SQL CODE

mysql&gt; SELECT DAYOFMONTH('1998-02-03');
+---------------------------------------------------------+
| DAYOFMONTH('1998-02-03')                                |
+---------------------------------------------------------+
| 3                                                       |
+---------------------------------------------------------+
1 row in set (0.00 sec)

DAYOFWEEK(date)

Returns the weekday index for date (1 = Sunday, 2 = Monday, ., 7 = Saturday). These index values correspond to the ODBC standard.

SQL CODE

mysql&gt; SELECT DAYOFWEEK('1998-02-03');
+---------------------------------------------------------+
|DAYOFWEEK('1998-02-03')                                  |
+---------------------------------------------------------+
| 3                                                       |
+---------------------------------------------------------+
1 row in set (0.00 sec)

DAYOFYEAR(date)

Returns the day of the year for date, in the range 1 to 366.

SQL CODE

mysql&gt; SELECT DAYOFYEAR('1998-02-03');
+---------------------------------------------------------+
| DAYOFYEAR('1998-02-03')                                 |
+---------------------------------------------------------+
| 34                                                      |
+---------------------------------------------------------+
1 row in set (0.00 sec)

EXTRACT(unit FROM date)

The EXTRACT() function uses the same kinds of unit specifiers as DATE_ADD() or DATE_SUB(), but extracts parts from the date rather than performing date arithmetic.

SQL CODE

mysql&gt; SELECT EXTRACT(YEAR FROM '1999-07-02');
+---------------------------------------------------------+
| EXTRACT(YEAR FROM '1999-07-02')                         |
+---------------------------------------------------------+
| 1999                                                    |
+---------------------------------------------------------+
1 row in set (0.00 sec)

mysql&gt; SELECT EXTRACT(YEAR_MONTH FROM '1999-07-02 01:02:03');
+---------------------------------------------------------+
| EXTRACT(YEAR_MONTH FROM '1999-07-02 01:02:03')          |
+---------------------------------------------------------+
| 199907                                                  |
+---------------------------------------------------------+
1 row in set (0.00 sec)

FROM_DAYS(N)

Given a day number N, returns a DATE value.

SQL CODE

mysql&gt; SELECT FROM_DAYS(729669);
+---------------------------------------------------------+
| FROM_DAYS(729669)                                       |
+---------------------------------------------------------+
| 1997-10-07                                              |
+---------------------------------------------------------+
1 row in set (0.00 sec)

Note− Use FROM_DAYS() with caution on old dates. It is not intended for use with values that precede the advent of the Gregorian calendar (1582).

FROM_UNIXTIME(unix_timestamp)

FROM_UNIXTIME(unix_timestamp,format)

Returns a representation of theunix_timestampargument as a value in ‘YYYY-MM-DD HH:MM:SS or YYYYMMDDHHMMSS format, depending on whether the function is used in a string or in a numeric context. The value is expressed in the current time zone. The unix_timestamp argument is an internal timestamp values, which are produced by theUNIX_TIMESTAMP()function.

If the format is given, the result is formatted according to the format string, which is used in the same way as is listed in the entry for theDATE_FORMAT()function.

SQL CODE

mysql&gt; SELECT FROM_UNIXTIME(875996580);
+---------------------------------------------------------+
| FROM_UNIXTIME(875996580)                                |
+---------------------------------------------------------+
| 1997-10-04 22:23:00                                     |
+---------------------------------------------------------+
1 row in set (0.00 sec)

HOUR(time)

Returns the hour for time. The range of the return value is 0 to 23 for time-of-day values. However, the range of TIME values actually is much larger, so HOUR can return values greater than 23.

SQL CODE

mysql&gt; SELECT HOUR('10:05:03');
+---------------------------------------------------------+
| HOUR('10:05:03')                                        |
+---------------------------------------------------------+
| 10                                                      |
+---------------------------------------------------------+
1 row in set (0.00 sec)

LAST_DAY(date)

Takes a date or datetime value and returns the corresponding value for the last day of the month. Returns NULL if the argument is invalid.

SQL CODE

mysql&gt; SELECT LAST_DAY('2003-02-05');
+---------------------------------------------------------+
| LAST_DAY('2003-02-05')                                  |
+---------------------------------------------------------+
| 2003-02-28                                              |
+---------------------------------------------------------+
1 row in set (0.00 sec)

LOCALTIME and LOCALTIME()

LOCALTIME and LOCALTIME() are synonyms for NOW().

LOCALTIMESTAMP and LOCALTIMESTAMP()

LOCALTIMESTAMP and LOCALTIMESTAMP() are synonyms for NOW().

MAKEDATE(year,dayofyear)

Returns a date, given year and day-of-year values. The dayofyear value must be greater than 0 or the result will be NULL.

SQL CODE

mysql&gt; SELECT MAKEDATE(2001,31), MAKEDATE(2001,32);
+---------------------------------------------------------+
| MAKEDATE(2001,31), MAKEDATE(2001,32)                    |
+---------------------------------------------------------+
| '2001-01-31', '2001-02-01'                              |
+---------------------------------------------------------+
1 row in set (0.00 sec)

MAKETIME(hour,minute,second)

Returns a time value calculated from the hour, minute and second arguments.

SQL CODE

mysql&gt; SELECT MAKETIME(12,15,30);
+---------------------------------------------------------+
| MAKETIME(12,15,30)                                      |
+---------------------------------------------------------+
| '12:15:30'                                              |
+---------------------------------------------------------+
1 row in set (0.00 sec)

MICROSECOND(expr)

Returns the microseconds from the time or datetime expression (expr) as a number in the range from 0 to 999999.

SQL CODE

mysql&gt; SELECT MICROSECOND('12:00:00.123456');
+---------------------------------------------------------+
| MICROSECOND('12:00:00.123456')                          |
+---------------------------------------------------------+
| 123456                                                  |
+---------------------------------------------------------+
1 row in set (0.00 sec)

MINUTE(time)

Returns the minute for time, in the range 0 to 59.

SQL CODE

mysql&gt; SELECT MINUTE('98-02-03 10:05:03');
+---------------------------------------------------------+
| MINUTE('98-02-03 10:05:03')                             |
+---------------------------------------------------------+
| 5                                                       |
+---------------------------------------------------------+
1 row in set (0.00 sec)

MONTH(date)

Returns the month for date, in the range 0 to 12.

SQL CODE

mysql&gt; SELECT MONTH('1998-02-03')
+---------------------------------------------------------+
| MONTH('1998-02-03')                                     |
+---------------------------------------------------------+
| 2                                                       |
+---------------------------------------------------------+
1 row in set (0.00 sec)

MONTHNAME(date)

Returns the full name of the month for a date.

SQL CODE

mysql&gt; SELECT MONTHNAME('1998-02-05');
+---------------------------------------------------------+
| MONTHNAME('1998-02-05')                                 |
+---------------------------------------------------------+
| February                                                |
+---------------------------------------------------------+
1 row in set (0.00 sec)

NOW()

Returns the current date and time as a value in ‘YYYY-MM-DD HH:MM:SS’ or YYYYMMDDHHMMSS format, depending on whether the function is used in a string or numeric context. This value is expressed in the current time zone.

SQL CODE

mysql&gt; SELECT NOW();
+---------------------------------------------------------+
| NOW()                                                   |
+---------------------------------------------------------+
| 1997-12-15 23:50:26                                     |
+---------------------------------------------------------+
1 row in set (0.00 sec)

PERIOD_ADD(P,N)

Adds N months to a period P (in the format YYMM or YYYYMM). Returns a value in the format YYYYMM. Note that the period argument P is not a date value.

SQL CODE

mysql&gt; SELECT PERIOD_ADD(9801,2);
+---------------------------------------------------------+
| PERIOD_ADD(9801,2)                                      |
+---------------------------------------------------------+
| 199803                                                  |
+---------------------------------------------------------+
1 row in set (0.00 sec)

PERIOD_DIFF(P1,P2)

Returns the number of months between periods P1 and P2. These periods P1 and P2 should be in the format YYMM or YYYYMM. Note that the period arguments P1 and P2 are not date values.

SQL CODE

mysql&gt; SELECT PERIOD_DIFF(9802,199703);
+---------------------------------------------------------+
| PERIOD_DIFF(9802,199703)                                |
+---------------------------------------------------------+
| 11                                                      |
+---------------------------------------------------------+
1 row in set (0.00 sec)

QUARTER(date)

Returns the quarter of the year for date, in the range 1 to 4.

SQL CODE

mysql&gt; SELECT QUARTER('98-04-01');
+---------------------------------------------------------+
| QUARTER('98-04-01')                                     |
+---------------------------------------------------------+
| 2                                                       |
+---------------------------------------------------------+
1 row in set (0.00 sec)

SECOND(time)

Returns the second for time, in the range 0 to 59.

SQL CODE

mysql&gt; SELECT SECOND('10:05:03');
+---------------------------------------------------------+
| SECOND('10:05:03')                                      |
+---------------------------------------------------------+
| 3                                                       |
+---------------------------------------------------------+
1 row in set (0.00 sec)

SEC_TO_TIME(seconds)

Returns the seconds argument, converted to hours, minutes and seconds, as a value in ‘HH:MM:SS’ or HHMMSS format, depending on whether the function is used in a string or numeric context.

SQL CODE

mysql&gt; SELECT SEC_TO_TIME(2378);
+---------------------------------------------------------+
| SEC_TO_TIME(2378)                                       |
+---------------------------------------------------------+
| 00:39:38                                                |
+---------------------------------------------------------+
1 row in set (0.00 sec)

STR_TO_DATE(str,format)

This is the inverse of the DATE_FORMAT() function. It takes a string str and a format string format. The STR_TO_DATE() function returns a DATETIME value if the format string contains both date and time parts. Else, it returns a DATE or TIME value if the string contains only date or time parts.

SQL CODE

mysql&gt; SELECT STR_TO_DATE('04/31/2004', '%m/%d/%Y');
+---------------------------------------------------------+
| STR_TO_DATE('04/31/2004', '%m/%d/%Y')                   |
+---------------------------------------------------------+
| 2004-04-31                                              |
+---------------------------------------------------------+
1 row in set (0.00 sec)

SUBDATE(date,INTERVAL expr unit) and SUBDATE(expr,days)

When invoked with the INTERVAL form of the second argument, SUBDATE() is a synonym for DATE_SUB(). For information on the INTERVAL unit argument, see the discussion for DATE_ADD().

SQL CODE

mysql&gt; SELECT DATE_SUB('1998-01-02', INTERVAL 31 DAY);
+---------------------------------------------------------+
| DATE_SUB('1998-01-02', INTERVAL 31 DAY)                 |
+---------------------------------------------------------+
| 1997-12-02                                              |
+---------------------------------------------------------+
1 row in set (0.00 sec)

mysql&gt; SELECT SUBDATE('1998-01-02', INTERVAL 31 DAY);
+---------------------------------------------------------+
| SUBDATE('1998-01-02', INTERVAL 31 DAY)                  |
+---------------------------------------------------------+
| 1997-12-02                                              |
+---------------------------------------------------------+
1 row in set (0.00 sec)

SUBTIME(expr1,expr2)

The SUBTIME() function returns expr1 . expr2 expressed as a value in the same format as expr1. The expr1 value is a time or a datetime expression, while the expr2 value is a time expression.

SQL CODE

mysql&gt; SELECT SUBTIME('1997-12-31 23:59:59.999999',
   -&gt; '1 1:1:1.000002');
+---------------------------------------------------------+
| SUBTIME('1997-12-31 23:59:59.999999'...                 |
+---------------------------------------------------------+
| 1997-12-30 22:58:58.999997                              |
+---------------------------------------------------------+
1 row in set (0.00 sec)

SYSDATE()

Returns the current date and time as a value in ‘YYYY-MM-DD HH:MM:SS’ or YYYYMMDDHHMMSS format, depending on whether the function is used in a string or in a numeric context.

SQL CODE

mysql&gt; SELECT SYSDATE();
+---------------------------------------------------------+
| SYSDATE()                                               |
+---------------------------------------------------------+
| 2006-04-12 13:47:44                                     |
+---------------------------------------------------------+
1 row in set (0.00 sec)

TIME(expr)

Extracts the time part of the time or datetime expressionexprand returns it as a string.

SQL CODE

mysql&gt; SELECT TIME('2003-12-31 01:02:03');
+---------------------------------------------------------+
| TIME('2003-12-31 01:02:03')                             |
+---------------------------------------------------------+
| 01:02:03                                                |
+---------------------------------------------------------+
1 row in set (0.00 sec)

TIMEDIFF(expr1,expr2)

The TIMEDIFF() function returns expr1 . expr2 expressed as a time value. These expr1 and expr2 values are time or date-and-time expressions, but both must be of the same type.

SQL CODE

mysql&gt; SELECT TIMEDIFF('1997-12-31 23:59:59.000001',
   -&gt; '1997-12-30 01:01:01.000002');
+---------------------------------------------------------+
| TIMEDIFF('1997-12-31 23:59:59.000001'.....              |
+---------------------------------------------------------+
|  46:58:57.999999                                        |
+---------------------------------------------------------+
1 row in set (0.00 sec)

TIMESTAMP(expr), TIMESTAMP(expr1,expr2)

With a single argument, this function returns the date or datetime expression expr as a datetime value. With two arguments, it adds the time expression expr2 to the date or datetime expressionexpr1and returns the result as a datetime value.

SQL CODE

mysql&gt; SELECT TIMESTAMP('2003-12-31');
+---------------------------------------------------------+
| TIMESTAMP('2003-12-31')                                 |
+---------------------------------------------------------+
| 2003-12-31 00:00:00                                     |
+---------------------------------------------------------+
1 row in set (0.00 sec)

TIMESTAMPADD(unit,interval,datetime_expr)

This function adds the integer expression interval to the date or datetime expressiondatetime_expr. The unit for interval is given by the unit argument, which should be one of the following values −

FRAC_SECOND
SECOND, MINUTE
HOUR, DAY
WEEK
MONTH
QUARTER or
YEAR

The unit value may be specified using one of the keywords as shown or with a prefix of SQL_TSI_.

For example, DAY and SQL_TSI_DAY both are legal.

SQL CODE

mysql&gt; SELECT TIMESTAMPADD(MINUTE,1,'2003-01-02');
+---------------------------------------------------------+
| TIMESTAMPADD(MINUTE,1,'2003-01-02')                     |
+---------------------------------------------------------+
| 2003-01-02 00:01:00                                     |
+---------------------------------------------------------+
1 row in set (0.00 sec)

TIMESTAMPDIFF(unit,datetime_expr1,datetime_expr2)

Returns the integer difference between the date or datetime expressions datetime_expr1 and datetime_expr2. The unit for the result is given by the unit argument. The legal values for the unit are the same as those listed in the description of the TIMESTAMPADD() function.

SQL CODE

mysql&gt; SELECT TIMESTAMPDIFF(MONTH,'2003-02-01','2003-05-01');
+---------------------------------------------------------+
| TIMESTAMPDIFF(MONTH,'2003-02-01','2003-05-01')          |
+---------------------------------------------------------+
| 3                                                       |
+---------------------------------------------------------+
1 row in set (0.00 sec)

TIME_FORMAT(time,format)

This function is used like the DATE_FORMAT() function, but the format string may contain format specifiers only for hours, minutes and seconds.

If the time value contains an hour part that is greater than 23, the %Hand %khour format specifiers produce a value larger than the usual range of 0 to 23. The other hour format specifiers produce the hour value modulo 12.

SQL CODE

mysql&gt; SELECT TIME_FORMAT('100:00:00', '%H %k %h %I %l');
+---------------------------------------------------------+
| TIME_FORMAT('100:00:00', '%H %k %h %I %l')              |
+---------------------------------------------------------+
| 100 100 04 04 4                                         |
+---------------------------------------------------------+
1 row in set (0.00 sec)

TIME_TO_SEC(time)

Returns the time argument converted to seconds.

SQL CODE

mysql&gt; SELECT TIME_TO_SEC('22:23:00');
+---------------------------------------------------------+
| TIME_TO_SEC('22:23:00')                                 |
+---------------------------------------------------------+
| 80580                                                   |
+---------------------------------------------------------+
1 row in set (0.00 sec)

TO_DAYS(date)

Given a date, returns a day number (the number of days since year 0).

SQL CODE

mysql&gt; SELECT TO_DAYS(950501);
+---------------------------------------------------------+
| TO_DAYS(950501)                                         |
+---------------------------------------------------------+
| 728779                                                  |
+---------------------------------------------------------+
1 row in set (0.00 sec)

UNIX_TIMESTAMP(), UNIX_TIMESTAMP(date)

If called with no argument, this function returns a Unix timestamp (seconds since ‘1970-01-01 00:00:00’ UTC) as an unsigned integer. If UNIX_TIMESTAMP() is called with a date argument, it returns the value of the argument as seconds since ‘1970-01-01 00:00:00’ UTC. date may be a DATE string, a DATETIME string, a TIMESTAMP, or a number in the format YYMMDD or YYYYMMDD.

SQL CODE

mysql&gt; SELECT UNIX_TIMESTAMP();
+---------------------------------------------------------+
| UNIX_TIMESTAMP()                                        |
+---------------------------------------------------------+
| 882226357                                               |
+---------------------------------------------------------+
1 row in set (0.00 sec)

mysql&gt; SELECT UNIX_TIMESTAMP('1997-10-04 22:23:00');
+---------------------------------------------------------+
| UNIX_TIMESTAMP('1997-10-04 22:23:00')                   |
+---------------------------------------------------------+
| 875996580                                               |
+---------------------------------------------------------+
1 row in set (0.00 sec)

UTC_DATE, UTC_DATE()

Returns the current UTC date as a value in ‘YYYY-MM-DD’ or YYYYMMDD format, depending on whether the function is used in a string or numeric context.

SQL CODE

mysql&gt; SELECT UTC_DATE(), UTC_DATE() + 0;
+---------------------------------------------------------+
| UTC_DATE(), UTC_DATE() + 0                              |
+---------------------------------------------------------+
| 2003-08-14, 20030814                                    |
+---------------------------------------------------------+
1 row in set (0.00 sec)

UTC_TIME, UTC_TIME()

Returns the current UTC time as a value in ‘HH:MM:SS’ or HHMMSS format, depending on whether the function is used in a string or numeric context.

SQL CODE

mysql&gt; SELECT UTC_TIME(), UTC_TIME() + 0;
+---------------------------------------------------------+
| UTC_TIME(), UTC_TIME() + 0                              |
+---------------------------------------------------------+
| 18:07:53, 180753                                        |
+---------------------------------------------------------+
1 row in set (0.00 sec)

UTC_TIMESTAMP, UTC_TIMESTAMP()

Returns the current UTC date and time as a value in ‘YYYY-MM-DD HH:MM:SS’ or in a YYYYMMDDHHMMSS format, depending on whether the function is used in a string or in a numeric context.

SQL CODE

mysql&gt; SELECT UTC_TIMESTAMP(), UTC_TIMESTAMP() + 0;
+---------------------------------------------------------+
| UTC_TIMESTAMP(), UTC_TIMESTAMP() + 0                    |
+---------------------------------------------------------+
| 2003-08-14 18:08:04, 20030814180804                     |
+---------------------------------------------------------+
1 row in set (0.00 sec)

WEEK(date[,mode])

This function returns the week number for date. The two-argument form of WEEK() allows you to specify whether the week starts on a Sunday or a Monday and whether the return value should be in the range from 0 to 53 or from 1 to 53. If the mode argument is omitted, the value of the default_week_format system variable is used

ModeFirst Day of weekRangeWeek 1 is the first week.0Sunday0-53with a Sunday in this year1Monday0-53with more than 3 days this year2Sunday1-53with a Sunday in this year3Monday1-53with more than 3 days this year4Sunday0-53with more than 3 days this year5Monday0-53with a Monday in this year6Sunday1-53with more than 3 days this year7Monday1-53with a Monday in this year

SQL CODE

mysql&gt; SELECT WEEK('1998-02-20');
+---------------------------------------------------------+
| WEEK('1998-02-20')                                      |
+---------------------------------------------------------+
| 7                                                       |
+---------------------------------------------------------+
1 row in set (0.00 sec)

WEEKDAY(date)

Returns the weekday index for date (0 = Monday, 1 = Tuesday, . 6 = Sunday).

SQL CODE

mysql&gt; SELECT WEEKDAY('1998-02-03 22:23:00');
+---------------------------------------------------------+
| WEEKDAY('1998-02-03 22:23:00')                          |
+---------------------------------------------------------+
| 1                                                       |
+---------------------------------------------------------+
1 row in set (0.00 sec)

WEEKOFYEAR(date)

Returns the calendar week of the date as a number in the range from 1 to 53. WEEKOFYEAR() is a compatibility function that is equivalent to WEEK(date,3).

SQL CODE

mysql&gt; SELECT WEEKOFYEAR('1998-02-20');
+---------------------------------------------------------+
| WEEKOFYEAR('1998-02-20')                                |
+---------------------------------------------------------+
| 8                                                       |
+---------------------------------------------------------+
1 row in set (0.00 sec)

YEAR(date)

Returns the year for date, in the range 1000 to 9999, or 0 for the .zero. date.

SQL CODE

mysql&gt; SELECT YEAR('98-02-03');
+---------------------------------------------------------+
| YEAR('98-02-03')                                        |
+---------------------------------------------------------+
| 1998                                                    |
+---------------------------------------------------------+
1 row in set (0.00 sec)

YEARWEEK(date), YEARWEEK(date,mode)

Returns the year and the week for a date. The mode argument works exactly like the mode argument to the WEEK() function. The year in the result may be different from the year in the date argument for the first and the last week of the year.

SQL CODE

mysql&gt; SELECT YEARWEEK('1987-01-01');
+---------------------------------------------------------+
| YEAR('98-02-03')YEARWEEK('1987-01-01')                  |
+---------------------------------------------------------+
| 198653                                                  |
+---------------------------------------------------------+
1 row in set (0.00 sec)

Note− The week number is different from what the WEEK() function would return (0) for optional arguments 0 or 1, as WEEK() then returns the week in the context of the given year.

SQL - Temporary Tables

What are Temporary Tables?

There are RDBMS, which support temporary tables. Temporary Tables are a great feature that lets youstore and process intermediate resultsby using the same selection, update, and join capabilities that you can use with typical SQL Server tables.

The temporary tables could be very useful in some cases to keep temporary data. The most important thing that should be known for temporary tables is that they will be deleted when the current client session terminates.

Temporary tables are available in MySQL version 3.23 onwards. If you use an older version of MySQL than 3.23, you can’t use temporary tables, but you can useheap tables.

As stated earlier, temporary tables will only last as long as the session is alive. If you run the code in a PHP script, the temporary table will be destroyed automatically when the script finishes executing. If you are connected to the MySQL database server through the MySQL client program, then the temporary table will exist until you close the client or manually destroy the table.

Example

Here is an example showing you the usage of a temporary table.

SQL CODE

mysql&gt; CREATE TEMPORARY TABLE SALESSUMMARY (
   -&gt; product_name VARCHAR(50) NOT NULL
   -&gt; , total_sales DECIMAL(12,2) NOT NULL DEFAULT 0.00
   -&gt; , avg_unit_price DECIMAL(7,2) NOT NULL DEFAULT 0.00
   -&gt; , total_units_sold INT UNSIGNED NOT NULL DEFAULT 0
);
Query OK, 0 rows affected (0.00 sec)

mysql&gt; INSERT INTO SALESSUMMARY
   -&gt; (product_name, total_sales, avg_unit_price, total_units_sold)
   -&gt; VALUES
   -&gt; ('cucumber', 100.25, 90, 2);

mysql&gt; SELECT * FROM SALESSUMMARY;
+--------------+-------------+----------------+------------------+
| product_name | total_sales | avg_unit_price | total_units_sold |
+--------------+-------------+----------------+------------------+
| cucumber     |      100.25 |          90.00 |                2 |
+--------------+-------------+----------------+------------------+
1 row in set (0.00 sec)

When you issue a SHOW TABLES command, then your temporary table will not be listed out in the list. Now, if you log out of the MySQL session and then issue a SELECT command, you will find no data available in the database. Even your temporary table will not be existing.

Dropping Temporary Tables

By default, all the temporary tables are deleted by MySQL when your database connection gets terminated. Still if you want to delete them in between, then you can do so by issuing aDROP TABLEcommand.

Following is an example on dropping a temporary table.

SQL CODE

mysql&gt; CREATE TEMPORARY TABLE SALESSUMMARY (
   -&gt; product_name VARCHAR(50) NOT NULL
   -&gt; , total_sales DECIMAL(12,2) NOT NULL DEFAULT 0.00
   -&gt; , avg_unit_price DECIMAL(7,2) NOT NULL DEFAULT 0.00
   -&gt; , total_units_sold INT UNSIGNED NOT NULL DEFAULT 0
);
Query OK, 0 rows affected (0.00 sec)

mysql&gt; INSERT INTO SALESSUMMARY
   -&gt; (product_name, total_sales, avg_unit_price, total_units_sold)
   -&gt; VALUES
   -&gt; ('cucumber', 100.25, 90, 2);

mysql&gt; SELECT * FROM SALESSUMMARY;
+--------------+-------------+----------------+------------------+
| product_name | total_sales | avg_unit_price | total_units_sold |
+--------------+-------------+----------------+------------------+
| cucumber     |      100.25 |          90.00 |                2 |
+--------------+-------------+----------------+------------------+
1 row in set (0.00 sec)
mysql&gt; DROP TABLE SALESSUMMARY;
mysql&gt;  SELECT * FROM SALESSUMMARY;
ERROR 1146: Table 'TUTORIALS.SALESSUMMARY' doesn't exist

SQL - Clone Tables

There may be a situation when you need an exact copy of a table and the CREATE TABLE … or the SELECT… commands does not suit your purposes because the copy must include the same indexes, default values and so forth.

If you are using MySQL RDBMS, you can handle this situation by adhering to the steps given below −

Use SHOW CREATE TABLE command to get a CREATE TABLE statement that specifies the source table’s structure, indexes and all.
Modify the statement to change the table name to that of the clone table and execute the statement. This way you will have an exact clone table.
Optionally, if you need the table contents copied as well, issue an INSERT INTO or a SELECT statement too.

Example

Try out the following example to create a clone table forTUTORIALS_TBLwhose structure is as follows −

Step 1− Get the complete structure about the table.

SQL CODE

SQL&gt; SHOW CREATE TABLE TUTORIALS_TBL \G; 
*************************** 1. row *************************** 
      Table: TUTORIALS_TBL 
Create Table: CREATE TABLE 'TUTORIALS_TBL' ( 
  'tutorial_id' int(11) NOT NULL auto_increment, 
  'tutorial_title' varchar(100) NOT NULL default '', 
  'tutorial_author' varchar(40) NOT NULL default '', 
  'submission_date' date default NULL, 
  PRIMARY KEY  ('tutorial_id'), 
  UNIQUE KEY 'AUTHOR_INDEX' ('tutorial_author') 
) TYPE = MyISAM 
1 row in set (0.00 sec)

Step 2− Rename this table and create another table.

SQL CODE

SQL&gt; CREATE TABLE `CLONE_TBL` ( 
  -&gt; 'tutorial_id' int(11) NOT NULL auto_increment, 
  -&gt; 'tutorial_title' varchar(100) NOT NULL default '', 
  -&gt; 'tutorial_author' varchar(40) NOT NULL default '', 
  -&gt; 'submission_date' date default NULL, 
  -&gt; PRIMARY KEY  (`tutorial_id'), 
  -> UNIQUE KEY 'AUTHOR_INDEX' ('tutorial_author') 
-&gt; ) TYPE = MyISAM; 
Query OK, 0 rows affected (1.80 sec)

Step 3− After executing step 2, you will clone a table in your database. If you want to copy data from an old table, then you can do it by using the INSERT INTO… SELECT statement.

SQL CODE

SQL&gt; INSERT INTO CLONE_TBL (tutorial_id, 
   -&gt;                        tutorial_title, 
   -&gt;                        tutorial_author, 
   -&gt;                        submission_date) 
   -&gt; SELECT tutorial_id,tutorial_title, 
   -&gt;        tutorial_author,submission_date, 
   -&gt; FROM TUTORIALS_TBL; 
Query OK, 3 rows affected (0.07 sec) 
Records: 3  Duplicates: 0  Warnings: 0

Finally, you will have an exact clone table as you wanted to have.

SQL - Sub Queries

A Subquery or Inner query or a Nested query is a query within another SQL query and embedded within the WHERE clause.

A subquery is used to return data that will be used in the main query as a condition to further restrict the data to be retrieved.

Subqueries can be used with the SELECT, INSERT, UPDATE, and DELETE statements along with the operators like =, <, >, >=, <=, IN, BETWEEN, etc.

There are a few rules that subqueries must follow −

Subqueries must be enclosed within parentheses.
A subquery can have only one column in the SELECT clause, unless multiple columns are in the main query for the subquery to compare its selected columns.
An ORDER BY command cannot be used in a subquery, although the main query can use an ORDER BY. The GROUP BY command can be used to perform the same function as the ORDER BY in a subquery.
Subqueries that return more than one row can only be used with multiple value operators such as the IN operator.
The SELECT list cannot include any references to values that evaluate to a BLOB, ARRAY, CLOB, or NCLOB.
A subquery cannot be immediately enclosed in a set function.
The BETWEEN operator cannot be used with a subquery. However, the BETWEEN operator can be used within the subquery.

Subqueries with the SELECT Statement

Subqueries are most frequently used with the SELECT statement. The basic syntax is as follows −

SQL CODE

SELECT column_name [, column_name ]
FROM   table1 [, table2 ]
WHERE  column_name OPERATOR
   (SELECT column_name [, column_name ]
   FROM table1 [, table2 ]
   [WHERE])

Example

Consider the CUSTOMERS table having the following records −

SQL CODE

+----+----------+-----+-----------+----------+
| ID | NAME     | AGE | ADDRESS   | SALARY   |
+----+----------+-----+-----------+----------+
|  1 | Ramesh   |  35 | Ahmedabad |  2000.00 |
|  2 | Khilan   |  25 | Delhi     |  1500.00 |
|  3 | kaushik  |  23 | Kota      |  2000.00 |
|  4 | Chaitali |  25 | Mumbai    |  6500.00 |
|  5 | Hardik   |  27 | Bhopal    |  8500.00 |
|  6 | Komal    |  22 | MP        |  4500.00 |
|  7 | Muffy    |  24 | Indore    | 10000.00 |
+----+----------+-----+-----------+----------+

Now, let us check the following subquery with a SELECT statement.

SQL CODE

SQL&gt; SELECT * 
   FROM CUSTOMERS 
   WHERE ID IN (SELECT ID 
         FROM CUSTOMERS 
         WHERE SALARY &gt; 4500) ;

This would produce the following result.

SQL CODE

+----+----------+-----+---------+----------+
| ID | NAME     | AGE | ADDRESS | SALARY   |
+----+----------+-----+---------+----------+
|  4 | Chaitali |  25 | Mumbai  |  6500.00 |
|  5 | Hardik   |  27 | Bhopal  |  8500.00 |
|  7 | Muffy    |  24 | Indore  | 10000.00 |
+----+----------+-----+---------+----------+

Subqueries with the INSERT Statement

Subqueries also can be used with INSERT statements. The INSERT statement uses the data returned from the subquery to insert into another table. The selected data in the subquery can be modified with any of the character, date or number functions.

The basic syntax is as follows.

SQL CODE

INSERT INTO table_name [ (column1 [, column2 ]) ]
   SELECT [ *|column1 [, column2 ]
   FROM table1 [, table2 ]
   [ WHERE VALUE OPERATOR ]

Example

Consider a table CUSTOMERS_BKP with similar structure as CUSTOMERS table. Now to copy the complete CUSTOMERS table into the CUSTOMERS_BKP table, you can use the following syntax.

SQL CODE

SQL&gt; INSERT INTO CUSTOMERS_BKP
   SELECT * FROM CUSTOMERS 
   WHERE ID IN (SELECT ID 
   FROM CUSTOMERS) ;

Subqueries with the UPDATE Statement

The subquery can be used in conjunction with the UPDATE statement. Either single or multiple columns in a table can be updated when using a subquery with the UPDATE statement.

The basic syntax is as follows.

SQL CODE

UPDATE table
SET column_name = new_value
[ WHERE OPERATOR [ VALUE ]
   (SELECT COLUMN_NAME
   FROM TABLE_NAME)
   [ WHERE) ]

Example

Assuming, we have CUSTOMERS_BKP table available which is backup of CUSTOMERS table. The following example updates SALARY by 0.25 times in the CUSTOMERS table for all the customers whose AGE is greater than or equal to 27.

SQL CODE

SQL&gt; UPDATE CUSTOMERS
   SET SALARY = SALARY * 0.25
   WHERE AGE IN (SELECT AGE FROM CUSTOMERS_BKP
      WHERE AGE &gt;= 27 );

This would impact two rows and finally CUSTOMERS table would have the following records.

SQL CODE

+----+----------+-----+-----------+----------+
| ID | NAME     | AGE | ADDRESS   | SALARY   |
+----+----------+-----+-----------+----------+
|  1 | Ramesh   |  35 | Ahmedabad |   125.00 |
|  2 | Khilan   |  25 | Delhi     |  1500.00 |
|  3 | kaushik  |  23 | Kota      |  2000.00 |
|  4 | Chaitali |  25 | Mumbai    |  6500.00 |
|  5 | Hardik   |  27 | Bhopal    |  2125.00 |
|  6 | Komal    |  22 | MP        |  4500.00 |
|  7 | Muffy    |  24 | Indore    | 10000.00 |
+----+----------+-----+-----------+----------+

Subqueries with the DELETE Statement

The subquery can be used in conjunction with the DELETE statement like with any other statements mentioned above.

The basic syntax is as follows.

SQL CODE

DELETE FROM TABLE_NAME
[ WHERE OPERATOR [ VALUE ]
   (SELECT COLUMN_NAME
   FROM TABLE_NAME)
   [ WHERE) ]

Example

Assuming, we have a CUSTOMERS_BKP table available which is a backup of the CUSTOMERS table. The following example deletes the records from the CUSTOMERS table for all the customers whose AGE is greater than or equal to 27.

SQL CODE

SQL&gt; DELETE FROM CUSTOMERS
   WHERE AGE IN (SELECT AGE FROM CUSTOMERS_BKP
      WHERE AGE &gt;= 27 );

This would impact two rows and finally the CUSTOMERS table would have the following records.

SQL CODE

+----+----------+-----+---------+----------+
| ID | NAME     | AGE | ADDRESS | SALARY   |
+----+----------+-----+---------+----------+
|  2 | Khilan   |  25 | Delhi   |  1500.00 |
|  3 | kaushik  |  23 | Kota    |  2000.00 |
|  4 | Chaitali |  25 | Mumbai  |  6500.00 |
|  6 | Komal    |  22 | MP      |  4500.00 |
|  7 | Muffy    |  24 | Indore  | 10000.00 |
+----+----------+-----+---------+----------+

SQL - Using Sequences

A sequence is a set of integers 1, 2, 3, … that are generated in order on demand. Sequences are frequently used in databases because many applications require each row in a table to contain a unique value and sequences provide an easy way to generate them.

This chapter describes how to use sequences in MySQL.

Using AUTO_INCREMENT column

The simplest way in MySQL to use sequences is to define a column as AUTO_INCREMENT and leave the rest to MySQL to take care.

Example

Try out the following example. This will create a table and after that it will insert a few rows in this table where it is not required to give a record ID because its auto-incremented by MySQL.

SQL CODE

mysql&gt; CREATE TABLE INSECT
   -&gt; (
   -&gt; id INT UNSIGNED NOT NULL AUTO_INCREMENT,
   -&gt; PRIMARY KEY (id),
   -&gt; name VARCHAR(30) NOT NULL, # type of insect
   -&gt; date DATE NOT NULL, # date collected
   -&gt; origin VARCHAR(30) NOT NULL # where collected
);
Query OK, 0 rows affected (0.02 sec)
mysql&gt; INSERT INTO INSECT (id,name,date,origin) VALUES
   -&gt; (NULL,'housefly','2001-09-10','kitchen'),
   -&gt; (NULL,'millipede','2001-09-10','driveway'),
   -&gt; (NULL,'grasshopper','2001-09-10','front yard');
Query OK, 3 rows affected (0.02 sec)
Records: 3  Duplicates: 0  Warnings: 0
mysql&gt; SELECT * FROM INSECT ORDER BY id;
+----+-------------+------------+------------+
| id | name        | date       | origin     |
+----+-------------+------------+------------+
|  1 | housefly    | 2001-09-10 | kitchen    |
|  2 | millipede   | 2001-09-10 | driveway   |
|  3 | grasshopper | 2001-09-10 | front yard |
+----+-------------+------------+------------+
3 rows in set (0.00 sec)

Obtain AUTO_INCREMENT Values

The LAST_INSERT_ID( ) is an SQL function, so you can use it from within any client that understands how to issue SQL statements. Otherwise PERL and PHP scripts provide exclusive functions to retrieve auto-incremented value of last record.

PERL Example

Use themysql_insertidattribute to obtain the AUTO_INCREMENT value generated by a query. This attribute is accessed through either a database handle or a statement handle, depending on how you issue the query. The following example references it through the database handle.

SQL CODE

$dbh-&gt;do ("INSERT INTO INSECT (name,date,origin)
VALUES('moth','2001-09-14','windowsill')");
my $seq = $dbh-&gt;{mysql_insertid};

PHP Example

After issuing a query that generates an AUTO_INCREMENT value, retrieve the value by calling themysql_insert_id( )function.

SQL CODE

mysql_query ("INSERT INTO INSECT (name,date,origin)
VALUES('moth','2001-09-14','windowsill')", $conn_id);
$seq = mysql_insert_id ($conn_id);

Renumbering an Existing Sequence

There may be a case when you have deleted many records from a table and you want to re-sequence all the records. This can be done by using a simple trick, but you should be very careful to do this and check if your table is having a join with another table or not.

If you determine that resequencing an AUTO_INCREMENT column is unavoidable, the way to do it is to drop the column from the table, then add it again.

The following example shows how to renumber the id values in the insect table using this technique.

SQL CODE

mysql&gt; ALTER TABLE INSECT DROP id;
mysql&gt; ALTER TABLE insect
   -&gt; ADD id INT UNSIGNED NOT NULL AUTO_INCREMENT FIRST,
   -&gt; ADD PRIMARY KEY (id);

Starting a Sequence at a Particular Value

By default, MySQL will start the sequence from 1, but you can specify any other number as well at the time of table creation.

The following code block has an example where MySQL will start sequence from 100.

SQL CODE

mysql&gt; CREATE TABLE INSECT
   -&gt; (
   -&gt; id INT UNSIGNED NOT NULL AUTO_INCREMENT = 100,
   -&gt; PRIMARY KEY (id),
   -&gt; name VARCHAR(30) NOT NULL, # type of insect
   -&gt; date DATE NOT NULL, # date collected
   -&gt; origin VARCHAR(30) NOT NULL # where collected
);

Alternatively, you can create the table and then set the initial sequence value with ALTER TABLE.

SQL CODE

mysql&gt; ALTER TABLE t AUTO_INCREMENT = 100;

SQL - Handling Duplicates

There may be a situation when you have multiple duplicate records in a table. While fetching such records, it makes more sense to fetch only unique records instead of fetching duplicate records.

The SQLDISTINCTkeyword, which we have already discussed is used in conjunction with the SELECT statement to eliminate all the duplicate records and by fetching only the unique records.

Syntax

The basic syntax of a DISTINCT keyword to eliminate duplicate records is as follows.

SQL CODE

SELECT DISTINCT column1, column2,.....columnN 
FROM table_name
WHERE [condition]

Example

Consider the CUSTOMERS table having the following records.

SQL CODE

+----+----------+-----+-----------+----------+
| ID | NAME     | AGE | ADDRESS   | SALARY   |
+----+----------+-----+-----------+----------+
|  1 | Ramesh   |  32 | Ahmedabad |  2000.00 |
|  2 | Khilan   |  25 | Delhi     |  1500.00 |
|  3 | kaushik  |  23 | Kota      |  2000.00 |
|  4 | Chaitali |  25 | Mumbai    |  6500.00 |
|  5 | Hardik   |  27 | Bhopal    |  8500.00 |
|  6 | Komal    |  22 | MP        |  4500.00 |
|  7 | Muffy    |  24 | Indore    | 10000.00 |
+----+----------+-----+-----------+----------+

First, let us see how the following SELECT query returns duplicate salary records.

SQL CODE

SQL&gt; SELECT SALARY FROM CUSTOMERS
   ORDER BY SALARY;

This would produce the following result where the salary of 2000 is coming twice which is a duplicate record from the original table.

SQL CODE

+----------+
| SALARY   |
+----------+
|  1500.00 |
|  2000.00 |
|  2000.00 |
|  4500.00 |
|  6500.00 |
|  8500.00 |
| 10000.00 |
+----------+

Now, let us use the DISTINCT keyword with the above SELECT query and see the result.

SQL CODE

SQL&gt; SELECT DISTINCT SALARY FROM CUSTOMERS
   ORDER BY SALARY;

This would produce the following result where we do not have any duplicate entry.

SQL CODE

+----------+
| SALARY   |
+----------+
|  1500.00 |
|  2000.00 |
|  4500.00 |
|  6500.00 |
|  8500.00 |
| 10000.00 |
+----------+

SQL - Injection

If you take a user input through a webpage and insert it into a SQL database, there is a chance that you have left yourself wide open for a security issue known as theSQL Injection. This chapter will teach you how to help prevent this from happening and help you secure your scripts and SQL statements in your server side scripts such as a PERL Script.

Injection usually occurs when you ask a user for input, like their name and instead of a name they give you a SQL statement that you will unknowingly run on your database. Never trust user provided data, process this data only after validation; as a rule, this is done byPattern Matching.

In the example below, thenameis restricted to the alphanumerical characters plus underscore and to a length between 8 and 20 characters (modify these rules as needed).

SQL CODE

if (preg_match("/^\w{8,20}$/", $_GET['username'], $matches)) {
   $result = mysql_query("SELECT * FROM CUSTOMERS 
      WHERE name = $matches[0]");
} else {
   echo "user name not accepted";
}

To demonstrate the problem, consider this excerpt −

SQL CODE

// supposed input
$name = "Qadir'; DELETE FROM CUSTOMERS;";
mysql_query("SELECT * FROM CUSTOMSRS WHERE name='{$name}'");

The function call is supposed to retrieve a record from the CUSTOMERS table where the name column matches the name specified by the user. Under normal circumstances,$namewould only contain alphanumeric characters and perhaps spaces, such as the string ilia. But here, by appending an entirely new query to $name, the call to the database turns into disaster; the injected DELETE query removes all records from the CUSTOMERS table.

Fortunately, if you use MySQL, themysql_query()function does not permit query stacking or executing multiple SQL queries in a single function call. If you try to stack queries, the call fails.

However, other PHP database extensions, such asSQLiteandPostgreSQLhappily perform stacked queries, executing all the queries provided in one string and creating a serious security problem.

Preventing SQL Injection

You can handle all escape characters smartly in scripting languages like PERL and PHP. The MySQL extension for PHP provides the functionmysql_real_escape_string()to escape input characters that are special to MySQL.

SQL CODE

if (get_magic_quotes_gpc()) {
   $name = stripslashes($name);
}
$name = mysql_real_escape_string($name);
mysql_query("SELECT * FROM CUSTOMERS WHERE name='{$name}'");

The LIKE Quandary

To address the LIKE quandary, a custom escaping mechanism must convert user-supplied ’%’ and ’_’ characters to literals. Useaddcslashes(), a function that lets you specify a character range to escape.

SQL CODE

$sub = addcslashes(mysql_real_escape_string("%str"), "%_");
// $sub == \%str\_
mysql_query("SELECT * FROM messages 
   WHERE subject LIKE '{$sub}%'");

SQL - Database Tunning

It takes time to become a Database Expert or an expert Database Administrator. This all comes with lot of experience in various database designs and good trainings.

But the following list may be helpful for the beginners to have a nice database performance −

Use 3BNF database design explained in this tutorial in RDBMS Concepts chapter.
Avoid number-to-character conversions because numbers and characters compare differently and lead to performance downgrade.
While using SELECT statement, only fetch whatever information is required and avoid using * in your SELECT queries because it would load the system unnecessarily.
Create your indexes carefully on all the tables where you have frequent search operations. Avoid index on the tables where you have less number of search operations and more number of insert and update operations.
A full-table scan occurs when the columns in the WHERE clause do not have an index associated with them. You can avoid a full-table scan by creating an index on columns that are used as conditions in the WHERE clause of an SQL statement.
Be very careful of equality operators with real numbers and date/time values. Both of these can have small differences that are not obvious to the eye but that make an exact match impossible, thus preventing your queries from ever returning rows.
Use pattern matching judiciously. LIKE COL% is a valid WHERE condition, reducing the returned set to only those records with data starting with the string COL. However, COL%Y does not further reduce the returned results set since %Y cannot be effectively evaluated. The effort to do the evaluation is too large to be considered. In this case, the COL% is used, but the %Y is thrown away. For the same reason, a leading wildcard %COL effectively prevents the entire filter from being used.
Fine tune your SQL queries examining the structure of the queries (and subqueries), the SQL syntax, to discover whether you have designed your tables to support fast data manipulation and written the query in an optimum manner, allowing your DBMS to manipulate the data efficiently.
For queries that are executed on a regular basis, try to use procedures. A procedure is a potentially large group of SQL statements. Procedures are compiled by the database engine and then executed. Unlike an SQL statement, the database engine need not optimize the procedure before it is executed.
Avoid using the logical operator OR in a query if possible. OR inevitably slows down nearly any query against a table of substantial size.
You can optimize bulk data loads by dropping indexes. Imagine the history table with many thousands of rows. That history table is also likely to have one or more indexes. When you think of an index, you normally think of faster table access, but in the case of batch loads, you can benefit by dropping the index(es).
When performing batch transactions, perform COMMIT at after a fair number of records creation in stead of creating them after every record creation.
Plan to defragment the database on a regular basis, even if doing so means developing a weekly routine.

Built-In Tuning Tools

Oracle has many tools for managing SQL statement performance but among them two are very popular. These two tools are −

Explain plan− tool identifies the access path that will be taken when the SQL statement is executed.
tkprof− measures the performance by time elapsed during each phase of SQL statement processing.

If you want to simply measure the elapsed time of a query in Oracle, you can use the SQL*Plus command SET TIMING ON.

Check your RDBMS documentation for more detail on the above-mentioned tools and defragmenting the database.