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

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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