Date: Tue, 6 Apr 2004 08:05:15 -0700 (PDT)
From: Stephan Szabo <sszabo@megazone.bigpanda.com>
To: Robert Treat <xzilla@users.sourceforge.net>
Cc: pgsql-sql@postgresql.org
Subject: Re: partial unique constraint
In-Reply-To: <1081261772.31785.48.camel@camel>
Message-ID: <20040406080241.G36214@megazone.bigpanda.com>
References: <1081261772.31785.48.camel@camel>
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On Tue, 6 Apr 2004, Robert Treat wrote:

> Trying to come up with the proper syntax to meet the following criteria:
>
> create table foo (bar integer, baz boolean UNIQUE (bar, baz = true));
>
> note the above syntax is not correct, but should demonstrate what i'm
> trying to do; I want to add a unique constraint such that we only allow
> one case of bar and baz = true...  i can have unlimited bar and baz =
> false, and there can be multiple bar and baz = true if the bars are
> different...  did some doc reading and mail list searching but a valid
> syntax for this seems to be escaping me...
>
> btw I'm pretty sure I could do this with an external trigger, but am
> wondering about a constraint oriented approach

You can't precisely with a constraint (AFAIR sql is marginally limited in
that way at least in 92 and 99), but you should be able to do it with a
partial unique index something like
 create unique index foo_i on foo(bar) where baz is true;