Re: multi-column unique constraints with nullable columns

Someone else suggested using coalesce to me as well. Isn't your function equivilant to mine? If so, I think the most elegant of these solutions is closer to the one I proposed (with Stephen's modification). Unfortunately, we've still got the concurrency problems that Stephen pointed out.

I sure would like the ability to affect the UNIQUE constraint's behavior to support this. Although, I'm probably in the minority and it would obviously be un-portable.

I think my plan will actually be to plug leaks in the domain layer and move toward non-nullable natural keys. I was hoping to find something better, but I frankly don't have the experience in house to write contstraints I'll have confidence in.

Thanks for the tip!
Phill

-----Original Message-----
From: Mikey [mailto:mikeboscia(at)gmail(dot)com]
Sent: Thu 5/5/2005 12:03 PM
To: Tornroth, Phill
Cc: pgsql-sql(at)postgresql(dot)org
Subject: Re: [SQL] multi-column unique constraints with nullable columns

It may be possible to bypass the NULL != NULL by using coalesce. Here
is an example:

<BEGIN SQL>

create table foo2 (
a integer not null,
b integer not null,
c integer null,
UNIQUE (a,b,c)
);

create function foo2_unique_func() RETURNS trigger AS '
DECLARE
isfound integer = 0;
BEGIN
isfound = (select count(*) from foo2 where
(new.a,new.b,coalesce(new.c::TEXT,''EmPtY'')) in (select
a,b,coalesce(c::TEXT,''EmPtY'')
from foo2));
RAISE NOTICE ''isfound: %'', isfound;
IF isfound > 0 THEN
RAISE EXCEPTION ''Columns a,b,c Must Be Unique values
(%,%,%)'', new.a, new.b, new.c;
ELSE
RETURN NEW;
END IF;
END; ' language 'plpgsql';

CREATE TRIGGER foo2_unique BEFORE INSERT OR UPDATE ON foo2
FOR EACH ROW EXECUTE PROCEDURE foo2_unique_func();

insert into foo2 values (1,300, null);
insert into foo2 values (1,300, null);

select * from foo2;
select * from foo2 where (1,300,coalesce(null::TEXT,'EmPtY')) in
(select a,b,coalesce(c::TEXT,'EmPtY') from foo2);

drop table foo2 cascade;
drop function foo2_unique_func() cascade;

<END SQL>