Date: Mon, 29 Jun 2009 10:02:32 +0200
From: "A. Kretschmer" <andreas.kretschmer@schollglas.com>
To: pgsql-sql@postgresql.org
Subject: Re: uniqueness constraint with NULLs
Message-ID: <20090629080232.GF24737@a-kretschmer.de>
References: <4A485431.3080707@cs.anu.edu.au>
	<20090629062602.GB24737@a-kretschmer.de>
	<4A486872.5060906@cs.anu.edu.au>
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In response to Robert Edwards :
> A. Kretschmer wrote:
> >In response to Robert Edwards :
> >>Can anyone suggest a way that I can impose uniqueness on a and b when
> >>c is NULL?
> >
> >Sure, use a functional index:
> >
> >test=# create table bobtest (a int, b int, c int);
> >CREATE TABLE
> >test=*# create unique index idx_bobtest on
> >bobtest(a,b,coalesce(c::text,'NULL'));
> >CREATE INDEX
> >test=*# insert into bobtest (a, b) values (1, 4);
> >INSERT 0 1
> >test=*# insert into bobtest (a, b, c) values (1, 4, NULL);
> >ERROR:  duplicate key value violates unique constraint "idx_bobtest"
> >test=!#
> >
> >
> >Regards, Andreas
> 
> Beautiful!

Btw:

if your data doesn't contains 0 or negative, you can use coalesce(c,0)
or coalesce(c,-1). It would be better for searching.

Andreas
-- 
Andreas Kretschmer
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