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

Chapter 7: Adding More to Our Queries
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point. An example would be where, for each row in a given table, you may have multiple results in the
subquery, but you’re looking for an action more complex than our IN operator provides. Essentially, what
I’m talking about here are situations where you wish you could use a JOIN operator on your subquery.
It’s at times like these that we turn to a somewhat lesser known construct in SQL — a derived table. A derived
table is made up of the columns and rows of a result set from a query. (Heck, they have columns, rows, data
types, and so on just like normal tables, so why not use them as such?)
Imagine for a moment that you want to get a list of account numbers and the associated territories for all
accounts that ordered a particular product — say, an HL Mountain Rear Wheel. No problem! Your query
might look something like this:
SELECT sc.AccountNumber, sst.Name
FROM Sales.SalesOrderHeader soh
JOIN Sales.SalesOrderDetail sod
ON soh.SalesOrderID = sod.SalesOrderID
JOIN Production.Product pp
ON sod.ProductID = pp.ProductID
JOIN Sales.Customer sc
ON sc.CustomerID = soh.CustomerID
JOIN Sales.SalesTerritory sst
ON sst.TerritoryID = sc.TerritoryID
WHERE pp.Name = ‘HL Mountain Rear Wheel’;
OK, so other than how many tables were required, that was easy. Now I’m going to throw you a twist —
let’s now say I want to know the account number and territory for all accounts that ordered not only an
HL Mountain Rear Wheel, but also an HL Mountain Front Wheel. Notice that I said they have to have
ordered both — now you have a problem. You’re first inclination might be to write something like:
WHERE pp.Name = ‘HL Mountain Rear Wheel’
AND pp.Name = ‘HL Mountain Front Wheel’
But that’s not going to work at all — each row is for a single product, so how can it have both HL Mountain
Rear Wheel and HL Mountain Front Wheel as the name at the same time? Nope — that’s not going
to get it at all (indeed, while it will run, you’ll never get any rows back).
What we really need here is to join the results of a query to find buyers of HL Mountain Rear Wheel
with the results of a query to find buyers of HL Mountain Front Wheel. How do we join results, however?
Well, as you might expect given the title of this section, through the use of derived tables.
To create our derived table, we need two things:
❑ To enclose our query that generates the result set in parentheses
❑ To alias the results of the query, so it can be referenced as a table
So, the syntax looks something like this:
SELECT
FROM () AS
JOIN