Re: UNIQUE boolean: only one row may be "true"

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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