@Sergei

Please correct me if I'm wrong but I'd always believed a primary key was enforced by a constraint which was implemented by means of both a UNIQUE index and NOT NULL constraints on all the participating columns.

The reason for my pedantry is nothing to do with metaphysics but all to do with foreign keys, and candidate keys. Hmmm. I've maybe got a few more QOTDs to do, even if I'm really just asking myself.