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

You’ll get a result that is a bit different from the one before:
-----
10
Warning: Null value is eliminated by an aggregate or other SET operation.
(1 row(s) affected)
This new result brings with it a question: why, since the AdditionalContactInfo column exists for
every row, is there a different COUNT for AdditionalContactInfo than there is for the row count in
general? The answer is fairly obvious when you stop to think about it — there isn’t a value, as such, for
the AdditionalContactInfo column in every row. In short, the COUNT, when used in any form other
than COUNT(*), ignores NULL values. Let’s verify that NULL values are the cause of the discrepancy:
SELECT COUNT(*)
FROM Person.Person
WHERE AdditionalContactInfo IS NULL;
This should yield the following record set:
-----------
19962
(1 row(s) affected)
Now let’s do the math:
10 + 19,962 = 19,972
Chapter 3: The Foundation Statements of T-SQL
That’s 10 records with a defined value in the AdditionalContactInfo field and 19,962 rows where the
value in the AdditionalContactInfo field is NULL, making a total of 19,972 rows.
Actually, all aggregate functions ignore NULLs except for COUNT(*). Think about this
for a minute — it can have a very significant impact on your results. Many users expect
NULL values in numeric fields to be treated as zero when performing averages, but a
NULL does not equal zero, and as such shouldn’t be used as one. If you perform an
AVG or other aggregate function on a column with NULLs, the NULL values will not be
part of the aggregation unless you manipulate them into a non-NULL value inside the
function (using COALESCE() or ISNULL(), for example). We’ll explore this further
in Chapter 7, but beware of this when coding in T-SQL and when designing your
database.
Why does it matter in your database design? Well, it can have a bearing on whether
you decide to allow NULL values in a field or not by thinking about the way that
queries are likely to be run against the database and how you want your aggregates
to work.
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