Question

I have realized that these two queries do not work if ‘fk_building’ is null. I don’t understand why, and cant seem to find a good explanation as it is kind of hard to google. Can someone explain why the hell !=3 doesnt return everything that is not 3??? including null rows? Why do I need to use <=> ?

update floor set fk_building = 3 where fk_building != 3 and floor_id = 1;

or

select * from floor where fk_building != 3

do not work where fk_building is null.

Answer 1

I would recommend looking at the “Common Mistakes” section on NULL values on Wikipedia.

From the entry:

For example, a WHERE clause or conditional statement might compare a column’s value with a constant. It is often incorrectly assumed that a missing value would be “less than” or “not equal to” a constant if that field contains Null, but, in fact, such expressions return Unknown

As users have suggested, you can use a null safe operator if your RDBMS allows it, or check for IS NULL.