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From: Bruno Wolff III <bruno@wolff.to>
To: Josh Berkus <josh@agliodbs.com>
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	Markus Bertheau <twanger@bluetwanger.de>, olly@lfix.co.uk,
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Subject: Re: multi column foreign key for implicitly unique columns
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On Wed, Aug 18, 2004 at 10:05:13 -0700,
  Josh Berkus <josh@agliodbs.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.