SQL Constraints
SQL constraints are used to identify rules for
the data in a table.
If there is any violation between the
constraint and the data action, the action is terminated by the constraint.
Constraints can be specified when the table is
created (inside the CREATE TABLE statement) or after the table is created
(inside the ALTER TABLE statement).
In SQL, we have the following constraints:
- NOT NULL - Indicates that a column cannot store NULL value
- UNIQUE - Ensures that each row for a column must have a unique value
- PRIMARY KEY - A combination of a NOT NULL and UNIQUE. Ensures that a column (or combination of two or more columns) have a unique identity which helps to find a particular record in a table more easily and quickly
- FOREIGN KEY - Ensure the referential integrity of the data in one table to match values in another table
- CHECK - Ensures that the value in a column meets a specific condition
- DEFAULT - Specifies a default value for a column
