MySQL Constraints

SQL constraints are used to specify rules for data in a table.

The constraint in MySQL is used to specify the rule that allows or restricts what values or data will be stored in the table. They provide a suitable method to ensure data accuracy and integrity inside the table. It also helps to limit the type of data that will be inserted inside the table. If any interruption occurs between the constraint and data action, the action is failed.

Create Constraints

Constraints can be specified when the table is created with the CREATE TABLE the statement, or after the table is created with the ALTER TABLE statement.

Syntax

MySQL Constraints

SQL constraints are used to specify rules for the data in a table.

Constraints 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 table. If there is any violation between the constraint and the data action, the action is aborted.

Constraints can be column level or table level. Column-level constraints apply to a column, and table-level constraints apply to the whole table.

The following constraints are commonly used in SQL:

• NOT NULL – Ensures that a column cannot have a NULL value
• UNIQUE – Ensures that all values in a column are different
• PRIMARY KEY – A combination of a NOT NULL and UNIQUE. Uniquely identifies each row in a table
• FOREIGN KEY – Prevents actions that would destroy links between tables
• CHECK – Ensures that the values in a column satisfy a specific condition
• DEFAULT – Sets a default value for a column if no value is specified
• CREATE INDEX – Used to create and retrieve data from the database very quickly