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To: Stephan Szabo <sszabo@megazone.bigpanda.com>
Cc: Richard Huxton <dev@archonet.com>,
	Markus Bertheau <twanger@bluetwanger.de>, pgsql-sql@postgresql.org
Subject: Re: multi column foreign key for implicitly unique columns 
In-reply-to: <20040817072443.S60684@megazone.bigpanda.com> 
References: <1092734724.2627.4.camel@dicaprio.akademie1.de>
	<4121D55D.1040105@archonet.com>
	<20040817072443.S60684@megazone.bigpanda.com>
Comments: In-reply-to Stephan Szabo <sszabo@megazone.bigpanda.com>
	message dated "Tue, 17 Aug 2004 07:26:56 -0700"
Date: Tue, 17 Aug 2004 10:46:35 -0400
Message-ID: <4307.1092753995@sss.pgh.pa.us>
From: Tom Lane <tgl@sss.pgh.pa.us>
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Stephan Szabo <sszabo@megazone.bigpanda.com> writes:
> No, actually, it's that the SQL92 (at least) spec says explicitly that
> there must be a unique constraint across all of the columns specified, not
> merely across a subset.

> "then the set of column names of that <reference column list> shall be
> equal to the set of column names in the unique columns of a unique
> constraint of the referenced table."

SQL99 says the same.  11.8 syntax rule 3a:

            a) If the <referenced table and columns> specifies a <reference
              column list>, then the set of <column name>s contained
              in that <reference column list> shall be equal to the
              set of <column name>s contained in the <unique column
              list> of a unique constraint of the referenced table.

I think one reason for this is that otherwise it's not clear which
unique constraint the FK constraint depends on.  Consider

	create table a (f1 int unique, f2 int unique);

	create table b (f1 int, f2 int,
			foreign key (f1,f2) references a(f1,f2));

How would you decide which constraint to make the FK depend on?
It'd be purely arbitrary.

			regards, tom lane