Re: multi column foreign key for implicitly unique columns

On Wed, Aug 18, 2004 at 10:05:13 -0700,
Josh Berkus <josh(at)agliodbs(dot)com> wrote:
>
> I have my own issue that forced me to use triggers. Given:
>
> table users (
> name
> login PK
> status
> etc. )
>
> table status (
> status
> relation
> label
> definition
> PK status, relation )
>
> the relationship is:
> users.status = status.status AND status.relation = 'users';
>
> This is a mathematically definable constraint, but there is no way in standard
> SQL to create an FK for it. This is one of the places I point to whenever
> we have the "SQL is imperfectly relational" discussion.

If users is supposed to reference status you can do this by adding a relation
column to users, using a constraint to force relation to always be 'users'
and then having (status, relation) being a foreign key.