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

know thus far? Actually, the very last query we ran has us 90 percent of the way there. Think about it for
a minute: an OUTER JOIN returns a NULL value in the ProductID column wherever there is no match.
What we are looking for is pretty much the same result set we received in the previous query, except that
we want to filter out any records that do have a ProductID, and we want only the special offer name. To
do this, we simply change our SELECT list and add an extra condition to the WHERE clause:
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
As expected, we have exactly the same stores that had NULL values before:
Description
------------------------------
Volume Discount over 60
(1 row(s) affected)
There is one question you might be thinking at the moment that I want to answer in anticipation, so that
you’re sure you understand why this will always work. The question is: “What if the discount record
really has a NULL value?” Well, that’s why we built a WHERE clause on the same field that was part of our
JOIN. If we are joining based on the stor_id columns in both tables, then only three conditions can exist:
❑ If the SpecialOfferProduct.SpecialOfferID column has a non-NULL value, then, according to the
ON operator of the JOIN clause, if a special offer record exists, then SpecialOffer.SpecialOfferID
must also have the same value as SpecialOfferProduct.SpecialOfferID (look at the ON ssop
.SpecialOfferID = sso.SpecialOfferID).
❑ If the SpecialOfferProduct.SpecialOfferID column has a non-NULL value, then, according
to the ON operator of the JOIN clause, if a special offer record does not exist, then SpecialOffer
.SpecialOfferID will be returned as NULL.
❑ If the SpecialOfferProduct.SpecialOfferID happens to have a NULL value, and SpecialOffer
.SpecialOfferID also has a NULL value, there will be no join (null does not equal null), and
SpecialOffer.SpecialOfferID will return NULL because there is no matching record.
A value of NULL does not join to a value of NULL. Why? Think about what we’ve already said about comparing
NULLs —a NULL does not equal NULL. Be extra careful of this when coding. One of the more
common questions I am asked is, “Why isn’t this working?” in a situation where people are using an
“equal to” operation on a NULL — it simply doesn’t work because they are not equal. If you want to test
this, try executing some simple code:
IF (NULL=NULL)
PRINT ‘It Does’
ELSE
PRINT ‘It Doesn’’t’
Chapter 4: JOINs
If you execute this, you’ll get the answer to whether your SQL Server thinks a NULL equals a NULL —
that is, it doesn’t.
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