Can you elaborate a little on what you want to do with this (as in, example
data, problem domain, etc.)?
This is probably not the answer you're looking for, but without knowing more,
I would guess that you need a different design, not a 1-true,many false field
constraint.
Perhaps you want something like this?
Table A:
thing_groupid (primary key)
...other group info...
Table B:
thing_groupid (reference to A.thing_groupid, many to 1)
thing_id
...details about thing...
?
MT
--- 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?
>
> 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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