Re: BUG #2334: WHERE IN (SUBSELECT) fails when column is null

On Fri, 17 Mar 2006, Patrick Narkinsky wrote:

> This may be expected behavior, but it certainly doesn't seem right to me,
> and it works as expected in sqlite.
>
> The database is as follows:
>
> BEGIN TRANSACTION;
> create table a (
> id integer,
> text varchar(20)
> );
> INSERT INTO a VALUES(0,'test');
> INSERT INTO a VALUES(1,'test2');
> create table b (
> id integer,
> a_id integer);
> INSERT INTO b VALUES(0,NULL);
> INSERT INTO b VALUES(1,NULL);
> INSERT INTO b VALUES(2,NULL);
> COMMIT;
>
> The following query returns everything in a in sqlite, but returns nothing
> in postgresql:
>
> select * from a where a.id not in (select a_id from b);

AFAICS, our behavior follows SQL.

a NOT IN b is NOT(a IN b)
IN is defined in terms of = ANY.
a =ANY (b) is basically (by my reading of 8.8 anyway):
True if a = bi for some bi in b
False if b is empty or a <> bi for all bi in b
Unknown otherwise
Since a <> NULL returns unknown, the second one won't come up, so the
whole expression won't ever be true after the negation. It might be false
or it might be unknown.