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

Chapter 4: JOINs
How It Works
Unlike when we were nonspecific about what columns we wanted (when we just used the *), this time
we were specific about what we wanted. Thus SQL Server knew exactly what to give us — the columns
have come out in exactly the order that we’ve specified in our SELECT list. Indeed, even adding in an
additional table, we were able to mix columns between all tables in the order desired.
How an INNER JOIN Is Like a WHERE Clause
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In the INNER JOINs that we’ve done so far, we’ve really been looking at the concepts that will work for
any JOIN type — the column ordering and aliasing is exactly the same for any JOIN. The part that
makes an INNER JOIN different from other JOINs is that it is an exclusive JOIN — that is, it excludes all
records that don’t have a value in both tables (the first named, or left table, and the second named, or
right table).
Our first example of this was shown in our imaginary Films and Actors tables. Bogey was left out
because he didn’t have a matching movie in the Films table. Let’s look at a real example or two to show
how this works.
While you probably haven’t realized it in the previous examples, we’ve already been working with the
exclusionary nature of the INNER JOIN. You see, the BusinessEntity table has many, many more rows
than the 290 or so that we’ve been working with. Indeed, our BusinessEntity table has information on
virtually any individual our company works with. They can be individuals associated with a particular
customer or vendor, or they can be employees. Let’s check this out by seeing how many rows exist in the
BusinessEntity table:
SELECT COUNT(*)
FROM Person.BusinessEntity
Run this and, assuming you haven’t added any new ones or deleted some, you should come up with
approximately 20,777 rows; that’s a lot more rows than the 290 that our Employee-related queries have
been showing us!
So where did those other 20,487 rows go? As expected, they were excluded from the Employee query
result set because there were no corresponding records in the Employee table. It is for this reason that an
INNER JOIN is comparable to a WHERE clause. Just as the WHERE clause limits the rows returned to those
that match the criteria specified, an INNER JOIN excludes rows because they have no corresponding
match in the other table.
Just for a little more proof and practice, let’s say we’ve been asked to produce a list of names associated
with at least one customer and the account number of the customers they are associated with. Consider
the following tables: