Re: database constraints

* Ben <bench@silentmedia.com> [2004-10-06 08:23:11 -0700]:

> If I have have the table:
>
> create table foo
> (
>     a int references bar(id),
>     b int references baz(id)
> )
>
> ... how do I make sure one and only one of the columns a and b are
> non-null? Is it even reasonable?

ALTER TABLE foo ADD CONSTRAINT must_have_a_or_b CHECK (a::int IS NULL
AND b::int IS NOT NULL OR a::int IS NOT NULL AND b::int IS NULL);

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