REFERENCES - Mailing list pgsql-sql

Hi,

i have a simple problem and wont request assistance fro m this list.
i have to tables

foo:id int8 UNIQUE

bar:id int8 UNIQUEfoo_id int8

Now i wont foo_id to be a id which exists in the foo, this could be done
with foo_id REFERENCES foo (id).
But what should i do if i want foo_id to be a id existing in foo or to
be zero(0).
I have created a view as "select id from foo UNION select 0" and tried
to REFERENCE against this view, but that
doesnt work.
Can this problem be solved by a somebody ????

Thanks

-- 
Andre Schubert            EMail:   Andre.Schubert@km3.de                         Tel:     03774 6625-78
km3 teledienst GmbH       Fax:     03774 6625-79