Mysql: where ..id != X doesn't include the NULL foreign key

Question

I have this Mysql table:

assignment: assignment_id (PK), company_id (FK), ...some other fields...

The FK company_id can be NULL if an assignment is not yet assigned to a company.

Now, I want to query all assignments that are not for company with id 2. So I did:

SELECT * FROM assignment WHERE company_id != 2

To my surprise, assignments with company_id NULL aren't returned in the results. I'd say that NULL != 2 so it should be returned. It works fine if I go with

(company_id IS NULL OR company_id != 2)

Can someone explain me the logic behind this? Does it have to do with the FK constraint?

Answer 1

The logic is simple: you can't really say whether or not NULL is equal to 2, because you don't know anything about NULL . Quoting the doc :

Conceptually, NULL means “a missing unknown value” and it is treated somewhat differently from other values. You cannot use arithmetic comparison operators such as =, <, or <> to test for NULL . [...] Because the result of any arithmetic comparison with NULL is also NULL , you cannot obtain any meaningful results from such comparisons.

You've already shown one possible workaround in your question, the other one is using <=> ( NULL-safe comparison operator ):

SELECT * FROM assignment
WHERE NOT(company_id <=> 2)

Answer 2

NULL is not a number. It is a completely different thing in mysql. So you can not compare something to NULL. To clear your confusion, in your case, all the fields where FK is not 2 are those fields where the "value" is other than 2. But when the value is NULL, it means there is no value assigned at all.

See MySql documentation