Question

I was writing a Stored procedure today, and I ran into a problem where if one of the values is null (whether it be from a SELECT statement or whether it be a passed in parameter) my expression would evaluate to false where it ought to evaluate to true.

SET ANSI_NULLS ON; DECLARE @1 INT; DECLARE @2 INT; DECLARE @3 INT; DECLARE @4 INT;  SET @1 = 1; SET @2 = NULL; SET @3 = 3; SET @4 = 3;  IF ((@1 <> @2) OR (@3 <> @4))    BEGIN      SELECT 1;    END ELSE    BEGIN      SELECT 2;    END SELECT @1, @2, @3, @4 

Returns:

2  1, NULL, 3, 3 

I expected it to return:

1  1, NULL, 3, 3 

I know I’m missing something simple, anybody have any idea what it is?

Related Question

SQL: Why are NULL Values filtered Out within this WHERE clause?

Answer 1

One way of dealing with this is you can wrap your NULL values in a known unexpected value, i.e. -1:

IF ((ISNULL(@1, -1) <> ISNULL(@2, -1)) OR (ISNULL(@3, -1) <> ISNULL(@4, -1)))    BEGIN      SELECT 1;    END ELSE    BEGIN      SELECT 2;    END 

Obviously if -1 was a possibility then use a different number. If there is no non-possible value, then you will have to use CASE statements.

A concise way of assigning NULL values to a ‘safe’ value follows:

SET @1 = ISNULL(@1, -1)

This allows the contional test code to remain ‘clutter-free’.