Introduction to SQL

Data Definition Language (DDL)

SQL can be termed as a DDL i.e. Data Definition Language. The DDL part in SQL enables it to create/delete/alter a database table. Through this DDL feature indexes, links between tables as well as constraints can be defined.

In this section we will be using commands that would enable us to CREATE, ALTER or DROP a database, table, index etc. This section also describes SELECT INTO which is used create a back up of a database table.

The commands have been described with appropriate example one at a time covering all the clauses.

SQL Create

Create a Database

CREATE statement can be used to create a database. A database is nothing but a collection of tables which are related to each other. The inter-relation amongst the tables in a database is created by attributes/fields which may be common to more than one table.

Syntax:

CREATE DATABASE database_name

Create Table

With CREATE TABLE; a new table can be created. A table can be considered as a basic structure within which data is stored. It consists of rows and columns. A row represents a single record while a column represents attributes/fields which can be thought of as one of the components contributing to make a record. While creating a table, data types have to be mentioned for each field.

Here are the most common Data Types:

The following statements/commands consist of DDL portion of SQL:

Create Table---- creates a new database table.

Alter Table : alters a database table.

Drop Table : deletes a database table.

Create Index : creates an index.

Drop Index : deletes an index.