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

Chapter 7: Adding More to Our Queries
This returns data that looks something like this:
AccountNumber OrderDate
------------- -----------------------
AW00000001 NULL
AW00000002 NULL
AW00000007 NULL
…
…
AW00030102 2002-12-01 00:00:00.000
AW00030104 2002-07-01 00:00:00.000
AW00030113 2002-09-01 00:00:00.000
(19820 row(s) affected)
Note that there are a couple of rows that have a NULL in the Order Date column. Why do you suppose
that is? The cause is, of course, because there is no record in the Orders table that matches the then current
record in the Customer table (the outer query).
This brings us to a small digression to take a look at a particularly useful function for this situation —
ISNULL().
Dealing with NULL Data — the ISNULL Function
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There are actually a few functions specifically meant to deal with NULL data, but the one of particular
use to us at this point is ISNULL(). ISNULL() accepts a variable (which we’ll talk about in Chapter 11)
or expression and tests it for a NULL value. If the value is indeed NULL, then the function returns some other
prespecified value. If the original value is not NULL, then the original value is returned. This syntax is
pretty straightforward:
ISNULL(, )
So, for example:
ISNULL Expression Value Returned
ISNULL(NULL, 5) 5
ISNULL(5, 15) 5
ISNULL(MyColumnName, 0) where MyColumnName IS NULL 0
ISNULL(MyColumnName, 0) where MyColumnName = 3 3
ISNULL(MyColumnName, 0) where MyColumnName =’Fred Farmer’ Fred Farmer