Question

declare @test int
declare @test2 int = 3

select case when @test != @test2 then 1 else 0 end
select case when @test = @test2 then 1 else 0 end

The result is always 0. Can someone explain this?

Answer 1

This behavior is controlled by the ANSI_NULLS set option. In fact if you SET ANSI_NULLS OFF you will get a 1 for the first select.

NULL is a special value, it is an unknown value. You can read your queries like this:

Select a value, when an unknown value is not equal to 3
Select a value, when an unknown value is equal to 3

Since the value is unknown it may or may not match, so you can’t assume that it’s not equal, as it is possible that they are equal. Since your comparing an unknown value it’s like your comparing all possible values, and since there is one possible value that invalidates it the statement is false.