Phil Factor (7/17/2014)
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.
But I'll write about it later in a separate post.
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.
If we're talking about foreign keys and candidate keys there is no diffeence between PK and UNIQUE.
I often use UNIQUE instead of PRIMARY KEY when I need to allow NULL into a PK column.