From: Peter Eisentraut <peter_e@gmx.net>
To: pgsql-sql@postgresql.org
Subject: Re: Foreign Unique Constraint
Date: Tue, 27 Mar 2007 17:58:42 +0200
User-Agent: KMail/1.9.5
Cc: "Jon Horsman" <horshaq@gmail.com>
References: <4f4c2a010703270621h5137a4e5rb515a9be9034a543@mail.gmail.com>
In-Reply-To: <4f4c2a010703270621h5137a4e5rb515a9be9034a543@mail.gmail.com>
MIME-Version: 1.0
Content-Type: text/plain;
  charset="iso-8859-1"
Content-Transfer-Encoding: 8bit
Content-Disposition: inline
Message-Id: <200703271758.42413.peter_e@gmx.net>

Jon Horsman wrote:
> Basically table 1 and table 2 both have the concept of an extension
> that must be unique but the rest of the info in the tables are
> different. 狢 need to ensure that if i add an entry to table 1 with
> extension 1000 that it will fail if there is already an entry in
> table2 with the same extension.

Make a third table that contains all the extension and add two columns, 
one referencing table 1 and one referencing table 2, and then add a 
constraint that only one of the two can be not null.

It's not very pretty, but you can use views to make the access simpler.

-- 
Peter Eisentraut
http://developer.postgresql.org/~petere/