Question

Considering the following table:

create table inttest (
    someint INT(10) NULL DEFAULT NULL
);

When I insert some random values

insert into inttest 
  (someint) 
values 
  (1),(2),(3),(1),(2),(NULL);

and execute the query

select *
  from inttest 
 where someint != 1;

MySQL returns 2,3,2 but not the NULL value. Is this correct? Should I extend my query with OR someint IS NULL or is it a bug in my MySQL installation?

Answer 1

SELECT 1 != NULL;
-- Output: NULL

Comparison operators return TRUE, FALSE or NULL.

You’re expecting NULL != 1 to give you TRUE but, sensibly, you get NULL for the comparison you make. This is because comparing anything with NULL is meaningless: NULL is not a value!.

Here’s a cunning trick whereby you could get the NULL in the resultset if you still really want it. The trick relies on reversing the logic, then manually excluding the NULL possibility:

SELECT * FROM `inttest`
 WHERE IF(`someint` = 1, FALSE, TRUE);

A more obvious approach might be:

SELECT * FROM `inttest`
 WHERE `someint` != '1'
    OR `someint` IS NULL;