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

Chapter 4: JOINs
16 Mountain-500 Silver Clearance 0.40 988
(244 row(s) affected)
If you were to perform a SELECT * against the discounts table, you’d quickly find that we have included
every row from that table except for SpecialOfferID 1, which we explicitly excluded from our results.
How It Works
We are doing a LEFT JOIN, and the SpecialOffer table is on the left side of the JOIN. But what
about the SpecialOfferProduct table? If we are joining and we don’t have a matching record for the
SpecialOfferProduct table, then what happens? Since it is not on the inclusive side of the JOIN (in
this case, the LEFT side), SQL Server will fill in a NULL for any value that comes from the opposite side
of the join if there is no match with the inclusive side of the JOIN. In this case, all but one of our rows
have ProductIDs. What we can discern from that is that all of our SpecialOffers are associated with
at least one product except one (SpecialOfferID 6).
We’ve answered the question then; of the 16 SpecialOffers available, only one is not being used (Volume
Discount over 60).
In this case, switching to a RIGHT JOIN would yield the same thing as the INNER JOIN, as the SpecialOfferProduct
table only contains rows where there is an active link between a special offer and a
product. The concept is, however, exactly the same. We could, for example, switch the order that we reference
the tables and then use a RIGHT JOIN.
Try It Out RIGHT OUTER JOINs
Now, let’s see what happens if we change the question to a RIGHT OUTER JOIN:
SELECT sso.SpecialOfferID, Description, DiscountPct, ProductID
FROM Sales.SpecialOfferProduct ssop
RIGHT OUTER JOIN Sales.SpecialOffer sso
ON ssop.SpecialOfferID = sso.SpecialOfferID
WHERE sso.SpecialOfferID != 1
How It Works
If we tried the preceding query with a LEFT JOIN, we would get back just the 243 rows we got with the
INNER JOIN. Run it as presented above, and we get back the unused special offer we wanted.
Finding Orphan or Non-Matching Records
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We can actually use the inclusive nature of OUTER JOINs to find non-matching records in the exclusive
table. What do I mean by that? Let’s look at an example.
Let’s change our special offer question. We want to know the store names for all the special offers that
are not associated with any products. Can you come up with a query to perform this based on what we