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

Chapter 6: Constraints
Ignoring Bad Data When You Create the Constraint
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All this syntax has been just fine for the circumstances in which you create the constraint at the same
time as you create the table. Quite often, however, data rules are established after the fact. Let’s say, for
instance, that you missed something when you were designing your database, and you now have some
records in an Invoicing table that show a negative invoice amount. You might want to add a rule that
won’t let any more negative invoice amounts into the database, but at the same time, you want to preserve
the existing records in their original state.
To add a constraint that won’t apply to existing data, you make use of the WITH NOCHECK option when
you perform the ALTER TABLE statement that adds your constraint. As always, let’s look at an example.
The Customers table we created in the Accounting database has a field called Phone. The Phone field
was created with a data type of char because we expected all of the phone numbers to be of the same
length. We also set it with a length of 15 in order to ensure that we have enough room for all the formatting
characters. However, we have not done anything to make sure that the records inserted into the
database do indeed match the formatting criteria that we expect. To test this out, we’ll insert a record in
a format that is not what we’re expecting, but might be a very honest mistake in terms of how someone
might enter a phone number:
INSERT INTO Customers
(CustomerName,
Address1,
Address2,
City,
State,
Zip,
Contact,
Phone,
FedIDNo,
DateInSystem)
VALUES
(‘MyCust’,
‘123 Anywhere’,
‘’,
‘Reno’,
‘NV’,
80808,
‘Joe Bob’,
‘555-1212’,
‘931234567’,
GETDATE());
Now let’s add a constraint to control the formatting of the Phone field:
ALTER TABLE Customers
ADD CONSTRAINT CN_CustomerPhoneNo
CHECK
(Phone LIKE ‘([0-9][0-9][0-9]) [0-9][0-9][0-9]-[0-9][0-9][0-9][0-9]‘);