From: Bruno Wolff III <bruno@wolff.to>
To: Randall Lucas <rlucas@tercent.net>
Cc: pgsql-sql@postgresql.org
Subject: Re: UNIQUE boolean: only one row may be "true"
Date: Tue, 6 May 2003 12:23:54 -0500
Message-ID: <20030506172354.GA21510@wolff.to> (raw)
In-Reply-To: <FA25F27B-7FDB-11D7-A71E-000A957653D6@tercent.net>
References: <FA25F27B-7FDB-11D7-A71E-000A957653D6@tercent.net>
On Tue, May 06, 2003 at 12:01:20 -0400,
Randall Lucas <rlucas@tercent.net> wrote:
>
> Hi folks,
>
> After a late night of SQL hacking, my brain fizzed out:
>
> How shall I create a constraint such that only one row may be 'true'?
> Rephrased, may I place a WHERE clause in a UNIQUE constraint, or
> alternatively, may I use a CHECK constraint with an aggregate?
You can't use a select in a check constraint.
Using a partial index seems to do what you want. Do something like:
create unique index thing_idx on thing (thing_group_id, is_main_thing)
where is_main_thing = true;
>
> Example:
>
> CREATE TABLE thing (
> thing_id serial primary key,
> thing_group_id int not null references thing_group(thing_group_id),
> is_main_thing_p boolean not null default 'f',
> -- there may be only one main thing per group:
> unique (thing_group_id, is_main_thing_p='t')
> -- or else something like:
> -- check (count (*) from thing where thing_group_id=NEW.thing_group_id
> and is_main_thing_p = 't' <2)
> );
>
> Best,
>
> Randall
>
>
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To: pgsql-sql@postgresql.org
Cc: bruno@wolff.to, rlucas@tercent.net
Subject: Re: UNIQUE boolean: only one row may be "true"
In-Reply-To: <20030506172354.GA21510@wolff.to>
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