SQL Sample Code - SQL Constraints - SQL Tutorial with Examples

SQL Constraints

Constraints are used to restrict the type of data that goes into a table.

There are a number of different types of ‘Constraints’;

Constraint	Description
NOT NULL	Enforces a column not to allow NULL values. (Not allowed to leave empty)
UNIQUE	Ensures all values in a column are different.
PRIMARY KEY	Primary key columns must contain unique values and a primary key column cannot contain NULL values. Only one column in a table can be a Primary Key
FOREIGN KEY	Used to ensure referential integrity of the data.
CHECK	Only allows values to be inserted that satisfy a defined criteria.
DEFAULT	Defines a default value for a column.

Go to the related Constraint to learn more and see examples.

The constraints above can be specified:

When creating a table – CREATE TABLE

When altering a table – ALTER TABLE

To learn more about CREATE TABLE and ALTER TABLE go to the relevant section.