From http://www.w3schools.com (Copyright Refsnes Data)
Constraints are used to limit the type of data that can go into a table.
Constraints can be specified when a table is created (with the CREATE TABLE statement) or after the table is created (with the ALTER TABLE statement).
We will focus on the following constraints:
The next chapters will describe each constraint in details.
From http://www.w3schools.com (Copyright Refsnes Data)