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

Chapter 7: Adding More to Our Queries
It should wind up looking like this:
SELECT Description
FROM Sales.SpecialOffer sso
WHERE sso.SpecialOfferID != 1
AND sso.SpecialOfferID NOT IN
(SELECT SpecialOfferID FROM Sales.SpecialOfferProduct);
This yields exactly the same record.
Correlated Subqueries
Two words for you on this section: Pay attention! This is another one of those little areas that, if you truly
get it, can really set you apart from the crowd. By “get it,” I don’t just mean that you understand how it
works, but also that you understand how important it can be.
Correlated subqueries are one of those things that make the impossible possible. What’s more, they often
turn several lines of code into one and create a corresponding increase in performance. The problem with
them is that they require a substantially different style of thought than you’re probably used to. Correlated
subqueries are probably the single easiest concept in SQL to learn, understand, and then promptly
forget, because the concept simply goes against the grain of how you think. If you’re one of the few who
choose to remember it as an option, then you will be one of the few who figure out that hard-to-figure-out
problem. You’ll also be someone with a far more complete tool set when it comes to squeezing every
ounce of performance out of your queries.
How Correlated Subqueries Work
What makes correlated subqueries different from the nested subqueries we’ve been looking at is that the
information travels in two directions rather than one. In a nested subquery, the inner query is processed
only once, and that information is passed out for the outer query, which will also execute just once —
essentially providing the same value or list that you would have provided if you had typed it in yourself.
With correlated subqueries, however, the inner query runs on information provided by the outer query,
and vice versa. That may seem a bit confusing (that chicken or the egg thing again), but it works in a
three-step process:
1. The outer query obtains a record and passes it into the inner query.
2. The inner query executes based on the passed-in value(s).
3. The inner query then passes the values from its results back to the outer query, which uses them
to finish its processing.
Correlated Subqueries in the WHERE Clause
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I realize that this is probably a bit confusing, so let’s look at it in an example.