SQL CONSTRAINT

You can place constraints to limit the type of data that can go into a table. Such constraints can be specified when the table when the table is first created via the CREATE TABLE statement, or after the table is already created via the ALTER T ABLE statement.

Common types of constraints include the following:

- NOT NULL
- UNIQUE
- CHECK
- Primary Key
- Foreign Key

Each is described in detail below.

NOT NULL

By default, a column can hold NULL. If you not want to allow NULL value in a column, you will want to place a constraint on this column specifying that NULL is now not an allowable value.

For example, in the following statement,

CREATE TABLE Customer 
(SID integer NOT NULL, 
Last_Name varchar (30) NOT NULL, 
First_Name varchar(30));

Columns "SID" and "Last_Name" cannot include NULL, while "First_Name" can include NULL.

UNIQUE

The UNIQUE constraint ensures that all values in a column are distinct.

For example, in the following statement,

CREATE TABLE Customer 
(SID integer Unique, 
Last_Name varchar (30), 
First_Name varchar(30));

Column "SID" cannot include duplicate values, while such constraint does not hold for columns "Last_Name" and "First_Name".

Please note that a column that is specified as a primary key must also be unique. At the same time, a column that's unique may or may not be a primary key.

CHECK

The CHECK constraint ensures that all values in a column satisfy certain conditions.

For example, in the following statement,

CREATE TABLE Customer 
(SID integer CHECK (SID > 0), 
Last_Name varchar (30), 
First_Name varchar(30));

Column "SID" must only include integers greater than 0.

Please note that the CHECK constraint does not get enforced by MySQL at this time.