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

Note that whether you use a LEFT or a RIGHT JOIN doesn’t matter as long as the correct
table or group of tables is on the corresponding side of the JOIN. For example,
we could have run the preceding query using a RIGHT JOIN as long as we also
switched which sides of the JOIN the Person and Employee tables were on. For
example, this would have yielded exactly the same results:
SELECT pp.BusinessEntityID, pp.FirstName, pp.LastName
FROM HumanResources.Employee hre
RIGHT OUTER JOIN Person.Person pp
ON pp.BusinessEntityID = hre.BusinessEntityID
WHERE hre.BusinessEntityID IS NULL
When we take a look at even more advanced queries, we’ll run into a slightly more popular way of finding
records that exist in one table without there being corresponding records in another table. Allow me to
preface that by saying that using JOINs is usually our best bet in terms of performance. There are exceptions
to the rule that we will cover as we come across them, but in general, the use of JOINs will be best
when faced with multiple options.
Dealing with More Complex OUTER JOINs
Now we’re on to our second illustration and how to make use of it. This scenario is all about dealing
with an OUTER JOIN mixed with some other JOIN (no matter what the variety).
It is when combining an OUTER JOIN with other JOINs that the concept of sides becomes even more critical.
What’s important to understand here is that everything to the “left” — or before — the JOIN in
question will be treated just as if it were a single table for the purposes of inclusion or exclusion from the
query. The same is true for everything to the “right” — or after — the JOIN. The frequent mistake here is
to perform a LEFT OUTER JOIN early in the query and then use an INNER JOIN late in the query. The
OUTER JOIN includes everything up to that point in the query, but the INNER JOIN may still create a
situation where something is excluded! My guess is that you will, like most people (including me for
a while), find this exceptionally confusing at first, so let’s see what we mean with some examples. Because
none of the databases that come along with SQL Server has any good scenarios for demonstrating this,
we’re going to have to create a database and sample data of our own.
If you want to follow along with the examples, the example database called Chapter4DB can be created
by running Chapter4DB.sql from the downloadable source code. Simply open the file in the Management
Studio query window and execute it.
Again, in order to utilize the next several examples, you must execute the
Chapter4DB.sql script included in the downloadable code for this book.
Chapter 4: JOINs
What we are going to do is to build up a query step-by-step and watch what happens. The query we are
looking for will return a vendor name and the address of that vendor. The example database only has a
few records in it, so let’s start out by selecting all the choices from the central item of the query — the
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