You can enforce a particular formatting requirement. For example, if a table has a column for phone numbers you may want to ensure that users enter them all in the same manner. For North American phone numbers, you could use a constraint such as:

CHECK ( Phone LIKE '(___) ___-____' );

Once this CHECK condition is in place, if you attempt to set a Phone value to 9835, for example, the change is not allowed.

For example, you can add a constraint on the Employees table to ensure that the TerminationDate is always later than, or equal to, the StartDate:

CHECK(TerminationDate >= StartDate);

• Choosing constraints
• Using CHECK constraints on columns
• Using CHECK constraints on tables