Question

So I have a simple query that returns a listing of products

SELECT     Model, CategoryID
FROM         Products
WHERE     (Model = '010-00749-01') 

This returns

010-00749-01    00000000-0000-0000-0000-000000000000
010-00749-01    NULL

Which is correct, so I wanted only the products whose CategoryID is not ‘00000000-0000-0000-0000-000000000000’ so I have

SELECT     Model, CategoryID
FROM         Products
WHERE     (Model = '010-00749-01') 
AND (CategoryID <> '00000000-0000-0000-0000-000000000000') 

But this returns no result. So I changed the query to

SELECT     Model, CategoryID
FROM         Products
WHERE     (Model = '010-00749-01') 
AND ((CategoryID <> '00000000-0000-0000-0000-000000000000') OR  (CategoryID  IS NULL))

Which returns expected result

010-00749-01    NULL

Can someone explain this behavior to me?
MS SQL Server 2008

Answer 1

Check out the full reference on Books Online – by default ANSI_NULLS is on meaning you’d need to use the approach you have done. Otherwise, you could switch that setting OFF at the start of the query to switch the behaviour round.

When SET ANSI_NULLS is ON, a SELECT
statement that uses WHERE column_name
= NULL returns zero rows even if there are null values in column_name. A
SELECT statement that uses WHERE
column_name <> NULL returns zero rows
even if there are nonnull values in
column_name.

…

When SET ANSI_NULLS
is ON, all comparisons against a null
value evaluate to UNKNOWN. When SET
ANSI_NULLS is OFF, comparisons of all
data against a null value evaluate to
TRUE if the data value is NULL.

Here’s a simple example to demonstrate the behaviour with regard to comparisons against NULL:

-- This will print TRUE
SET ANSI_NULLS OFF;
IF NULL <> 'A'
    PRINT 'TRUE'
ELSE
    PRINT 'FALSE'

-- This will print FALSE
SET ANSI_NULLS ON;
IF NULL <> 'A'
    PRINT 'TRUE'
ELSE
    PRINT 'FALSE'