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

Building a Nested Subquery
A nested subquery is one that goes in only one direction, returning either a single value for use in the outer query,
or perhaps a full list of values to be used with the IN operator. In the event you want to use an explicit =
operator, then you’re going to be using a query that returns a single value — that means one column from
one row. If you are expecting a list back, then you’ll need to use the IN operator with your outer query.
In the loosest sense, your query syntax is going to look something like one of these two syntax templates:
Or:
SELECT
FROM
WHERE = (
SELECT
FROM
WHERE )
SELECT
FROM
WHERE IN (
SELECT
FROM
[WHERE )]
Obviously, the exact syntax will vary, not only because you will be substituting the select list and exact
table names, but also because you may have a multitable join in either the inner or outer queries — or both.
Nested Queries Using Single-Value SELECT Statements
Let’s get down to the nitty-gritty with an explicit example. Let’s say, for example, that we wanted to
know the ProductIDs of every item sold on the first day any product was purchased from the system.
If you already know the first day that an order was placed in the system, then it’s no problem; the query
would look something like this:
SELECT DISTINCT sod.ProductID
FROM Sales.SalesOrderHeader soh
JOIN Sales.SalesOrderDetail sod
ON soh.SalesOrderID = sod.SalesOrderID
WHERE OrderDate = ‘07/01/2001’; --This is first OrderDate in the system
This yields the correct results:
ProductID
-----------
707
708
709
…
…
776
777
778
(47 row(s) affected)
Chapter 7: Adding More to Our Queries
189