From: "Leif B. Kristensen" <leif@solumslekt.org>
To: pgsql-sql@postgresql.org
Subject: Re: Need help with 'unique parents' constraint
Date: Mon, 12 Sep 2005 20:02:34 +0200
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On Sunday 11 September 2005 16:04, Greg Sabino Mullane wrote:

> Not just old-fashioned, it's the biological law! (among homo sapiens
> anyway). I'd approach this with a trigger, as you can do complex
> checks and get back nice customized error messages. A sample script
> follows. Hard to tell without seeing your whole schema, but I see no
> need for a relation_id primary key if you already have a unique
> constraint on child_fk and parent_fk, so I made those into the
> primary key for the relations table:

Thank you for an excellent answer. I think I will have to study your 
code for a while. But is it such a bad idea to have a separate column 
for the primary key here? I see that there are two schools on this, 
with diametrically opposed views. For my own part, I feel that it at 
least doesn't hurt to have a surrogate key. Secondly, a single key 
value is easier to reference from another table than a composite key.
-- 
Leif Biberg Kristensen
http://solumslekt.org/