Re: A pk is *both* a physical and a logical object.

From: DBMS_Plumber <>
Date: Sat, 14 Jul 2007 23:19:36 -0700
Message-ID: <>

On Jul 13, 5:00 am, "David Cressey" <> wrote:
> By bringing the "SQL school of data management" into the discussion, I did
> not intend to "appropriate" the concept of unique identifier (key, or
> candidate key) nor even the concept of "primary key". I specifically did
> NOT intend the inference that in order to advocate the selection and use of
> a primary key, one had to be of the SQL school of data management.
> The only reason I brought SQL oriented people into the discussion was to
> describe a specific reason for naming one of the keys to be primary, a
> reason that may or may not be grounded in relational theory. It's my
> perception that the concept of primary key is not in fact grounded in
> relational theory, and that relational theory gets along just as well
> without it. I wanted either confirmation or refutation of that perception
> from this group.

 As a footnote, Quel had/has a primary key / foreign key concept. As did the data language GEM.

 It wasn't just the SQL community who thought picking a single key was a practical and useful idea.

 And Jan's absolutely correct. There is no theoretical justification for the idea of a primary key as distinct from a simple candidate key at all. Received on Sun Jul 15 2007 - 08:19:36 CEST

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