? about Composite Keys + ON DELETE/UPDATE SET NULL

Hi,

Is there any way to effect behavior similar to the following:

FOREIGN KEY (field1, field2)

REFERENCES table2 (field1, field2)

ON UPDATE CASCADE

ON DELETE (SET field2 = NULL) -- leaving field1 with whatever value is
currently holds

With MATCH SIMPLE the NULL in field2 is sufficient to "break" the Foreign
Key and let the record on table2 become deleted.

Given the general disdain for composite keys I can see why this particular
behavior has been overlooked but it does seem reasonable, in the presence of
"MATCH SIMPLE", to specify that only some of the constrained fields be
affected by "ON DELETE SET NULL".

Some syntax like:

ON DELETE SET NULL(fieldn [, fieldn+1 .]) would seem to be the most
declarative way to accomplish this.

Alternatively, having the ability to fire a trigger function would make
custom behavior possible since the trigger function could just do a
"NEW.field2 = NULL" and then return NEW.

Like: ON DELETE CALL trigger_function();

In my particular use-case I have a field on the FK table (invoice number)
and, in the presence of a (store id) it wants to enforce that the physical
invoice exists for that particular store. Should the physical invoice
become deleted I want to still leave the (invoice number) present but set
the (store id) back to NULL. I know, and can consider, other possibilities
but the first thing that came to mind was using "ON DELETE SET NULL(field2)"
and so I figure I might as well toss it out here and see what others think.

Thanks,

David J.