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

While this works just fine, queries of this type almost always fall into the category of those that can be
done using an inner join rather than a nested SELECT. For example, we could get the same results as the
preceding subquery by running this simple join:
SELECT DISTINCT pp.ProductID, Name
FROM Production.Product pp
JOIN Sales.SpecialOfferProduct ssop
ON pp.ProductID = ssop.ProductID;
For performance reasons, you want to use the join method as your default solution if you don’t have a
specific reason for using the nested SELECT — we’ll discuss this more before the chapter’s done.
SQL Server is actually pretty smart about this kind of thing. In the lion’s share of situations, SQL
Server will actually resolve the nested subquery solution to the same query plan it would use on the
join — indeed, if you checked the query plan for both the nested subquery and the previous join, you’d
find it was the exact same plan. So, with that in mind, the truth is that most of the time, there really
isn’t that much difference. The problem, of course, is that I just said most of the time. When the query
plans vary, the join is usually the better choice, and thus the recommendation to use that syntax by
default.
Using a Nested SELECT to Find Orphaned Records
This type of nested SELECT is nearly identical to our previous example, except that we add the NOT operator.
The difference this makes when you are converting to join syntax is that you are equating to an
outer join rather than an inner join.
Before we do the nested SELECT syntax, let’s review one of our examples of an outer join from Chapter 4.
In this query, we were trying to identify all the special offers that do not have matching products:
SELECT Description
FROM Sales.SpecialOfferProduct ssop
RIGHT OUTER JOIN Sales.SpecialOffer sso
ON ssop.SpecialOfferID = sso.SpecialOfferID
WHERE sso.SpecialOfferID != 1
AND ssop.SpecialOfferID IS NULL;
This returned one row:
Description
---------------------
Volume Discount over 60
(1 row(s) affected)
Chapter 7: Adding More to Our Queries
This is the way that, typically speaking, things should be done (or as a LEFT JOIN). I can’t say, however,
that it’s the way that things are usually done. The join usually takes a bit more thought, so we usually
wind up with the nested SELECT instead.
See if you can write this nested SELECT on your own. Once you’re done, come back and take a look.
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