Data constraints

A
constraint is a limitation that you place on the data that users can enter into
a column or group of columns. A constraint is part of the table definition; you
can implement constraints when you create the table or later. You can remove a
constraint from the table without affecting the table or the data, and you can
temporarily disable certain constraints.

Constraints
make it possible to further restrict the domain of an attribute. A domain
describes the set of possible values for a given attribute, and can be
considered a constraint on the value of the attribute. For instance, a
constraint can restrict a given integer attribute to values between 1 and 10.

Types of constraints

There are also some
standard constraints that are intrinsic in most of the DBMSs. These are;

Constraint name	Description
Primary key	Designates a column or combination of columns as Primary Key and therefore, values of columns cannot be repeated or left blank.
Foreign key	Relates one table with another table.
Unique	Specifies that values of a column or combination of columns cannot be repeated.
Not null	Specifies that a column cannot contain empty values.
Check	Specifies a condition which each row of a table must satisfy.