Beginning Microsoft SQL Server 2008 ... - S3 Tech Training

Chapter 6: Constraints
CHECK Constraints
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The nice thing about CHECK constraints is that they are not restricted to a particular column. They can be
related to a column, but they can also be essentially table related in that they can check one column
against another, as long as all the columns are within a single table and the values are for the same row
being updated or inserted. They can also check that any combination of column values meets a criterion.
The constraint is defined using the same rules that you would use in a WHERE clause. Examples of the
criteria for a CHECK constraint include:
Goal SQL
Limit Month column to
appropriate numbers
BETWEEN 1 AND 12
Proper SSN formatting LIKE ‘[0-9][0-9][0-9]-[0-9][0-9]-[0-9][0-9][0-9][0-9]’
Limit to a specific list of
Shippers
Price must be positive UnitPrice >= 0
Referencing another
column in the same row
IN (‘UPS’, ‘Fed Ex’, ‘USPS’)
ShipDate >= OrderDate
This really only scratches the surface, and the possibilities are virtually endless. Almost anything you
could put in a WHERE clause, you can also put in your constraint. What’s more, CHECK constraints are
very fast performance-wise as compared to the alternatives (rules and triggers).
Still building on our Accounting database, let’s add a modification to our Customers table to check for a
valid date in our DateInSystem field (you can’t have a date in the system that’s in the future):
ALTER TABLE Customers
ADD CONSTRAINT CN_CustomerDateInSystem
CHECK
(DateInSystem