NULL values in where clause

Question

i've got a table "bla" like this:

[id]    [name]    [fk]
1       test      4
2       foo       5
3       bar       NULL

if i do the sql query

SELECT * FROM bla WHERE fk <> 4

i only get the record with the id 2. i don't get the record with id 3 where fk is null. I thought NULL != 4. Seems that this is wrong.

Why is this so?

Answer 1

NULL doesn't compare equal to anything. You'll need to accept nulls explicitly:

where fk <> 4 or fk is null;

See Working with NULL for more information about NULL handling.

Answer 2

NULL is special in that it represents an "unknown" value. This can't be compared to numbers (or any other value for that matter), hence the result -

Is NULL <> 4? The answer is - don't know. Is 4 different from an unknown value?

Try this instead:

SELECT * FROM bla WHERE fk <> 4 OR FK IS NULL