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

Chapter 7: Adding More to Our Queries
deleted.Year,
deleted.Month,
deleted.ProductID,
deleted.QtySold;
This time we see more than one action and, in the case of UPDATED results, we have data for both the
inserted (the new values) and deleted (the old values) tables:
$action Year Month ProductID QtySold Year Month ProductID QtySold
---------- ------ ----- ----------- ----------- ------ ----- ----------- ----------
INSERT 2003 8 928 2 NULL NULL NULL NULL
INSERT 2003 8 882 1 NULL NULL NULL NULL
UPDATE 2003 8 707 249 2003 8 707 242
…
…
UPDATE 2003 8 963 32 2003 8 963 31
UPDATE 2003 8 970 54 2003 8 970 53
UPDATE 2003 8 998 139 2003 8 998 138
(38 row(s) affected)
Performance Considerations
We’ve already touched on some of the macro-level “what’s the best thing to do” stuff as we’ve gone
through the chapter, but, like most things in life, it’s not as easy as all that. What I want to do here is provide
something of a quick reference for performance issues for your queries. I’ll try to steer you toward
the right kind of query for the right kind of situation.
Yes, it’s time again folks for one of my now famous soapbox diatribes. At issue this
time is the concept of blanket use of blanket rules.
What we’re going to be talking about in this section is the way that things usually
work. The word “usually” is extremely operative here. There are very few rules in
SQL that will be true 100 percent of the time. In a world full of exceptions, SQL has
to be at the pinnacle of that — exceptions are a dime a dozen when you try to describe
the performance world in SQL Server.
In short, you need to gauge just how important the performance of a given query is.
If performance is critical, then don’t take these rules too seriously — instead, use
them as a starting point, and then TEST, TEST, TEST!!!
JOINs versus Subqueries versus ?
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This is that area I mentioned earlier in the chapter that I had a heated debate with a coworker over. And,
as you might expect when two people have such conviction in their points of view, both of us were correct
up to a point (and, it follows, wrong up to a point).