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

Determining Which Tables and Data Types Use a Given
Rule or Default
If you ever go to delete or alter your rules or defaults, you may first want to take a look at which tables
and data types are making use of them. Again, SQL Server comes to the rescue with a system-stored procedure.
This one is called sp_depends. Its syntax looks like this:
EXEC sp_depends
sp_depends provides a listing of all the objects that depend on the object you’ve requested information
about.
Triggers for Data Integrity
We’ve got a whole chapter coming up on triggers, but any discussion of constraints, rules, and defaults
would not be complete without at least a mention of triggers.
One of the most common uses of triggers is to implement data integrity rules. Since we have that chapter
coming up, I’m not going to get into it very deeply here, other than to say that triggers have a very large
number of things they can do data integrity–wise that a constraint or rule could never hope to do. The
downside (and you knew there had to be one) is that they incur substantial additional overhead and are,
therefore, much (very much) slower in almost any circumstance. They are procedural in nature (which is
where they get their power), but they also happen after everything else is done and should be used only
as a relatively last resort.
Choosing What to Use
Chapter 6: Constraints
Unfortunately, sp_depends is not a sure bet to tell you about every object that
depends on a parent object. SQL Server supports something called “deferred name
resolution.” Basically, deferred name resolution means that you can create objects
(primary stored procedures) that depend on another object — even before the second
(target of the dependency) object is created. For example, SQL Server will now
allow you to create a stored procedure that refers to a table even before the said table
is created. In this instance, SQL Server isn’t able to list the table as having a dependency
on it. Even after you add the table, it will not have any dependency listing if
you use sp_depends.
Wow. Here you are with all these choices, and now how do you figure out which is the right one to use?
Some of the constraints are fairly independent (PRIMARY and FOREIGN KEYs, UNIQUE constraints) — you
are using either them or nothing. The rest have some level of overlap with each other, and it can be rather
confusing when deciding what to use. You’ve gotten some hints from me as we’ve been going through
this chapter about what some of the strengths and weaknesses are of each of the options, but it will probably
make a lot more sense if we look at them all together for a bit.
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