Re: multi column foreign key for implicitly unique columns

Josh Berkus 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.    

It'd be nice to say something like:

ALTER TABLE status ADD CONSTRAINT user_status_fk
FOREIGN KEY (status) WHERE relation = 'users'
REFERENCES users(status);

And the flip-side so you can have:

ALTER TABLE cheque_details ADD CONSTRAINT chq_trans_fk
FOREIGN KEY (trans_id)
REFERENCES transactions(trans_id) WHERE trans_type='CHQ';

Actually, since we can have a "unique index with where" this second form 
should be do-able shouldn't it?

--   Richard Huxton  Archonet Ltd