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

7
Adding More to Our Queries
When I first started writing about SQL Server a number of years ago, I was faced with the question
of when exactly to introduce more complex queries into the knowledge mix — this book faces that
question all over again. At issue is something of a “chicken or egg” thing — talk about scripting,
variables, and the like first, or get to some things that a beginning user might make use of long
before they do server-side scripting. This time around, the notion of more queries early won out.
Some of the concepts in this chapter are going to challenge you with a new way of thinking. You
already had a taste of this just dealing with joins, but you haven’t had to deal with the kind of depth
that I want to challenge you with in this chapter. Even if you don’t have that much procedural programming
experience, the fact is that your brain has a natural tendency to break complex problems
down into their smaller subparts (sub-procedures, logical steps) as opposed to solving them whole
(the “set,” or SQL way).
While SQL Server 2008 supports procedural language concepts now more than ever, my challenge
to you is to try and see the question as a whole first. Be certain that you can’t get it in a single
query. Even if you can’t think of a way, quite often you can break it up into several small queries
and then combine them one at a time back into a larger query that does it all in one task. Try to see
it as a whole and, if you can’t, then go ahead and break it down, but then combine it into the
whole again to the largest extent that makes sense.
This is really what’s at the heart of my challenge of a new way of thinking — conceptualizing the
question as a whole rather than in steps. When we program in most languages, we usually work
in a linear fashion. With SQL, however, you need to think more in terms of set theory. You can
liken this to math class and the notion of A union B, or A intersect B. We need to think less in terms
of steps to resolve the data and more about how the data fits together.
In this chapter, we’re going to be using this concept of data fit to ask what amounts to multiple questions
in just one query. Essentially, we’re going to look at ways of taking what seem like multiple
queries and placing them into something that will execute as a complete unit. In addition, we’ll
also be taking a look at query performance and what you can do to get the most out of queries.