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

6
Constraints
You’ve heard me talk about them, but now it’s time to look at them seriously — it’s time to deal
with constraints. We’ve talked a couple of times already about what constraints are, but let’s review
in case you decided to skip straight to this chapter.
A constraint is a restriction. Placed at either column or table level, a constraint
ensures that your data meets certain data integrity rules.
This goes back to the notion that I talked about back in Chapters 1 and 2, that ensuring data integrity
is not the responsibility of the programs that use your database, but rather the responsibility of the
database itself. If you think about it, this is really cool. Data is inserted, updated, and deleted from
the database by many sources. Even in stand-alone applications (situations where only one program
accesses the database), the same table may be accessed from many different places in the program. It
doesn’t stop there though. Your database administrator (that might mean you if you’re a dual-role
kind of person) may be altering data occasionally to deal with problems that arise. In more complex
scenarios, you can actually run into situations where literally hundreds of different access paths
exist for altering just one piece of data, let alone your entire database.
Moving the responsibility for data integrity into the database itself has been revolutionary to database
management. There are still many different things that can go wrong when you are attempting
to insert data into your database, but your database is now proactive rather than reactive to problems.
Many problems with what programs allow into the database are now caught much earlier in the
development process because, although the client program allowed the data through, the database
knows to reject it. How does it do it? Primarily with constraints (data types and triggers are among
the other worker bees of data integrity).
In this chapter, we’ll be looking at the three different types of constraints at a high level:
❑ Entity constraints
❑ Domain constraints
❑ Referential integrity constraints