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

Adding a DEFAULT Constraint to an Existing Table
While this one is still pretty much more of the same, there is a slight twist. We make use of our ALTER
statement and ADD the constraint as before, but we add a FOR operator to tell SQL Server what column is
the target for the DEFAULT:
ALTER TABLE Customers
ADD CONSTRAINT CN_CustomerDefaultDateInSystem
DEFAULT GETDATE() FOR DateInSystem;
And an extra example:
ALTER TABLE Customers
ADD CONSTRAINT CN_CustomerAddress
DEFAULT ‘UNKNOWN’ FOR Address1;
As with all constraints except for a PRIMARY KEY, we are able to add more than one per table.
You can mix and match any and all of these constraints as you choose — just be careful
not to create constraints that have mutually exclusive conditions. For example, don’t have
one constraint that says that col1 > col2 and another one that says that col2 > col1.
SQL Server will let you do this, and you wouldn’t see the issues with it until runtime.
Disabling Constraints
Sometimes we want to eliminate constraints, either just for a time or permanently. It probably doesn’t take
much thought to realize that SQL Server must give us some way of deleting constraints, but SQL Server
also allows us to just deactivate a FOREIGN KEY or CHECK constraint while otherwise leaving it intact.
The concept of turning off a data integrity rule might seem rather ludicrous at first. I mean, why would
you want to turn off the thing that makes sure you don’t have bad data? First, realize that any reason for
turning off a data integrity rule is going to be a temporary thing (otherwise you would be dropping it
entirely). The usual reason to disable a data integrity rule is because you already have bad data. This
data usually falls into two categories:
❑ Data that’s already in your database when you create the constraint
❑ Data that you want to add after the constraint is already built
SQL Server allows us to turn the integrity check off long enough to deal with the bad data we want to
make an exception for, and then re-enable the integrity check later — all without physically removing
the data integrity check.
You cannot disable PRIMARY KEY or UNIQUE constraints.
Chapter 6: Constraints
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