Date: Wed, 27 Jul 2005 15:37:59 -0500
From: Bruno Wolff III <bruno@wolff.to>
To: Jeff Boes <jeff@endpoint.com>
Cc: pgsql-sql@postgresql.org
Subject: Re: Foreign key with check?
Message-ID: <20050727203759.GA3884@wolff.to>
Mail-Followup-To: Bruno Wolff III <bruno@wolff.to>,
	Jeff Boes <jeff@endpoint.com>, pgsql-sql@postgresql.org
References: <42E7E9B3.4080500@endpoint.com>
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On Wed, Jul 27, 2005 at 16:08:19 -0400,
  Jeff Boes <jeff@endpoint.com> wrote:
> Given a table like this:
> 
> create table primary (
>   a integer primary key,
>   b boolean
> );
> 
> And another like this:
> 
> create table secondary (
>   a integer,
>   some_other_fields
> );
> 
> 
> I would like a foreign key constraint on the "secondary" table that
> looks something like:
> 
> foreign key (a, true) references primary (a, b)
> 
> That is, a row in "secondary" is allowed to reference a row in "primary"
> if and only if that referenced row has (b = true).

If what you are saying is that all entries in secondary need to reference
a row in primary and in addition the referenced row must have b true, then
you can do this fairly simply using a bit of extra space.
Add a row b to secondary with a constraint that b = true. Then make the
foreign key reference on both a and b.