Structure Query Language (SQL)

SQL is the most popular querying language used to create, modify, retrieve and manipulate data from RDBMS (i.e. Relational Database Management System). Basically, SQL is composed of two categories:

• Data Manipulation Language (DML).
• Data Definition Language (DDL).

Data Manipulation Language is used for

• retrieval of information from the database.
• insertion of new information into the database.
• deletion of information in the database.
• modification of information in the database.

Queries used in DML are as

• SELECT – for querying the database.
• INSERT – it inserts new rows to the table.
• DELETE – removes a specified row from the table.
• MODIFY – modifies an existing row.

Data Definition Language is used to specify a database scheme as a set of definitions. DDL statements are compiled and stored in a special file called a data dictionary. The data dictionary contains metadata (i.e. data about data).The basic idea is to hide implementation details of the database schemes from the users.

Queries used in DDL are as

• CREATE DATABASE – allows user to create a database.
• CREATE TABLE – allows user to create a table definition in a database.
• DROP TABLE – removes a table from a database.
• ALTER TABLE – modifies the definition of a table in a database.