Re: Problem with "NOT IN (subquery)

On Sun, 13 Nov 2005, Steve SAUTETNER wrote:

> Hi,
>
> I have a table named "famille" whose structure and content is :
>
>  famille_code | famille_mere_famille_code |  famille_libelle  |
> famille_niveau
> --------------+---------------------------+-------------------+-------------
> ---
>  000000       |                           | Mhre              |

Is that a NULL famille_mere_famille_code?

> The first col is the family id and the second is the mother family id.
> I would get a list of all families that are never in the col n�2, so
> the families that aren't node but leaf.
>
> The query, i made was "SELECT * FROM famille WHERE famille_code NOT IN
> (SELECT DISTINCT famille_mere_famille_code FROM famille);"
>
> But the DB returns 0 records instead of 15. If i use a list instead of a
> subquery it works normaly but it's not easy to manage it like this.
>
> So if anyone can help me please ...

In the case where the subselect returns a NULL, the behavior of IN and NOT
IN is rather unfortunate. A NOT IN B is basically NOT(A IN B) and A IN B
is basically A =ANY B IIRC. However, A=ANY B only returns false if A = Bi
returns false for all Bi contained in B and A = NULL returns unknown, not
false, so NOT IN cannot return true if the subselect contains a NULL.

If that is a null above, probably the best solution is to exclude NULLs
from the subselect results.