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Chapter 7: Adding More to Our Queries
Among the topics we’ll be covering in this chapter are:
❑ Nested subqueries
❑ Correlated subqueries
❑ Derived tables
❑ Making use of the EXISTS operator
❑ MERGE
❑ Optimizing query performance
We’ll see how, using subqueries, we can make the seemingly impossible completely possible, and how
an odd tweak here and there can make a big difference in your query performance.
What Is a Subquer y?
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A subquery is a normal T-SQL query that is nested inside another query. Subqueries are created using
parentheses when you have a SELECT statement that serves as the basis for either part of the data or the
condition in another query.
Subqueries are generally used to fill one of a few needs:
❑ To break a query into a series of logical steps
❑ To provide a listing to be the target of a WHERE clause together with [IN|EXISTS|ANY|ALL]
❑ To provide a lookup driven by each individual record in a parent query
Some subqueries are very easy to think of and build, but some are extremely complex — it usually
depends on the complexity of the relationship between the inner (the sub) and outer (the top) queries.
It’s also worth noting that most subqueries (but definitely not all) can also be written using a join. In
places where you can use a join instead, the join is usually the preferable choice for a variety of reasons
we will continue to explore over the remainder of the book.
I once got into a rather lengthy debate (perhaps 20 or 30 e-mails flying back and forth, with examples,
reasons, and so on over a few days) with a coworker over the joins versus subqueries issue.
Traditional logic says to always use the join, and that was what I was pushing (due to experience rather
than traditional logic — you’ve already seen several places in this book where I’ve pointed out how traditional
thinking can be bogus). My coworker was pushing the notion that a subquery would actually
cause less overhead — I decided to try it out.
What I found was essentially (as you might expect) that we were both right in certain circumstances.
We will explore these circumstances fully toward the end of the chapter after you have a bit more
background.
Now that we know what a subquery theoretically is, let’s look at some specific types and examples of
subqueries.