Re: PRIMARY KEY on a *group* of columns imply that each

On Wed, 27 Apr 2005, Stephane Bortzmeyer wrote:

> On Tue, Apr 26, 2005 at 03:48:44PM -0500,
>  Scott Marlowe <smarlowe@g2switchworks.com> wrote
>  a message of 26 lines which said:
>
> > Here's a quote from the SQL1992 spec that's VERY clear:
>
> Yes, PostgreSQL is right and implement the standard. Now, what's the
> rationale for the standard? I understand it for a single column but,
> for several columns, it should be still possible to have different
> tuples, such as (3, NULL) and (5, NULL) for instance.

The case that they're trying to prevent is two tuples like (3, NULL) and
(3,NULL) since uniqueness alone doesn't prevent them both from being
inserted.