On Thu, 17 Aug 2006, Alexander Farber wrote:
> I have this strange problem that the following statement works:
NULLs are not your friends. :(
> phpbb=> select user_id, username from phpbb_users
> phpbb-> where user_id in (select ban_userid from phpbb_banlist);
> user_id | username
> ---------+----------
> 3 | La-Li
> (1 row)
>
>
> But the negative one returns nothing:
>
> phpbb=> select user_id, username from phpbb_users
> phpbb-> where user_id not in (select ban_userid from phpbb_banlist);
> user_id | username
> ---------+----------
> (0 rows)
Sadly, these two look like they would give you all the users rows, but
they don't because of the NULL ban_userid. When the subselect returns
NULL for at least one row, you fall into this sort of case.
x NOT IN (...) is equivalent to NOT(x IN (...)) which is
NOT(x = ANY (...))
x = ANY (...) is basically defined as
True if x = y is true for some y in the subselect
False if x = y is false for all y in the subselect
Unknown otherwise
Since x = NULL is unknown and not true or false, you fall into the last
case with your query and data.
> Eventhough there are 3 other users in the phpbb_users table:
>
> phpbb=> select user_id, username from phpbb_users;
> user_id | username
> ---------+-----------
> -1 | Anonymous
> 3 | La-Li
> 4 | Vasja
> 2 | Alex
> (4 rows)
>
> And there is only one user (La-Li, id=3) in the phpbb_banlist:
>
> phpbb=> select * from phpbb_banlist;
> ban_id | ban_userid | ban_ip | ban_email
> --------+------------+--------+-------------
> 1 | 3 | |
> 4 | | | *@gmail.com
> (2 rows)
