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

Now let’s see this at work in our query:
SELECT sc.AccountNumber,
ISNULL(CAST((SELECT Min(OrderDate)
FROM Sales.SalesOrderHeader soh
WHERE soh.CustomerID = sc.CustomerID) AS varchar), ‘NEVER ORDERED’)
AS OrderDate
FROM Sales.Customer sc;
Now, some example lines that we had problems with. We go from:
…
…
AW00000696 NULL
AW00000697 NULL
AW00000698 NULL
AW00011012 2003-09-17 00:00:00.000
AW00011013 2003-10-15 00:00:00.000
AW00011014 2003-09-24 00:00:00.000
…
…
to something a bit more useful:
…
…
AW00000696 NEVER ORDERED
AW00000697 NEVER ORDERED
AW00000698 NEVER ORDERED
AW00011012 Sep 17 2003 12:00AM
AW00011013 Oct 15 2003 12:00AM
AW00011014 Sep 24 2003 12:00AM
…
…
Notice that I also had to put the CAST() function into play to get this to work. The reason has to do
with casting and implicit conversion. Because the column Order Date is of type DateTime there is
an error generated since NEVER ORDERED can’t be converted to the DateTime data type. Keep CAST()
in mind — it can help you out of little troubles like this one. This is covered further later in this chapter.
So, at this point, we’ve seen correlated subqueries that provide information for both the WHERE clause
and for the select list. You can mix and match these two in the same query if you wish.
Derived Tables
Chapter 7: Adding More to Our Queries
Sometimes you need to work with the results of a query, but you need to work with the results of that
query in a way that doesn’t really lend itself to the kinds of subqueries that we’ve discussed up to this
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