SQL Constraints and Commands

• Constraints List

  Here are a few commonly used constraints:

  Constraint	Description
  NOT NULL	values cannot be NULL
  UNIQUE	values cannot match any older value
  PRIMARY KEY	used to uniquely identify a row
  FOREIGN KEY	references a row in another table

• SQL can perform many tasks such as

  • retrieving data
  • creating tables
  • inserting and changing rows
  • deleting tables
  • and so on

  Due to the wide range of tasks SQL can perform, it can be overwhelming to remember all the commands.

  To make it easier to understand different SQL commands, we can categorize SQL commands into several sublanguages:

• Data Query Language (DQL) - SELECT
• Data Manipulation Language (DML) - INSERT, UPDATE, DELETE
• Data Definition Language (DDL) - CREATE TABLE, CREATE DATABASE, ALTER TABLE, DROP TABLE
• Data Control Language (DCL) - GRANT, REVOKE
• Transaction Control Language (TCL) - SAVEPOINT, ROLLBACK, COMMIT