Re: [SQL] multi-column unique constraints with nullable columns

[Tornroth, Phill]

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:[EMAIL PROTECTED]
Sent: Thu 5/5/2005 12:03 PM
To: Tornroth, Phill
Cc: pgsql-sql@postgresql.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>




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