Martijn,

> > I only believed existed something like a NATURAL JOIN that performs
> > the JOIN using the only existing FK.
>
> FKs aren't in the database because of JOINs, they are in the database
> to enforce integrity.

Let's say they serve both purposes. :-)
BTW, natural joins are part of the SQL standard; I seem to remember that
they work as Matteo described (even though the NATURAL keyword, which
Firebird does not support, should be specified). Think of them as
"default join specifications".
Ciao
--
____
_/\/ando