SQL referred to as Structured Query Language) is a special-purpose programming language designed for managing data in relational database management systems (RDBMS).
Originally based upon relational algebra and tuple relational calculus its scope includes data insert, query, update and delete, schema creation and modification, and data access control.
SQL can be divided into two parts: The Data Manipulation Language (DML) and the Data Definition Language (DDL).
The query and update commands form the DML part of SQL:
SELECT – extracts data from a database
UPDATE – updates data in a database
DELETE – deletes data from a database
INSERT INTO – inserts new data into a database
The DDL part of SQL permits database tables to be created or deleted. It also defines indexes (keys), specifies links between tables, and imposes constraints between tables. The most important DDL statements in SQL are:
CREATE DATABASE – creates a new database
ALTER DATABASE – modifies a database
CREATE TABLE – creates a new table
ALTER TABLE – modifies a table
DROP TABLE – deletes a table
CREATE INDEX – creates an index (search key)
DROP INDEX – deletes an index