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

From: Cimode <>
Date: Sun, 15 Jul 2007 05:44:26 -0700
Message-ID: <>

On 15 juil, 08:19, DBMS_Plumber <> wrote:
> 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.
Quite frankly I do not believe there is such thing as a SQL school of 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.
I realize I may have given the wrong impression here. My intent never was to put a community on trial but rather to pinpoint the downfalls of listenning to one specific community to define the nature of unique identifer concept (whether legitimate or not). No matter what the theoretical standpoint of unique identifer currently is, the fact is that most SQL practitionners *do* have a bias toward defining *primary keys* and *unique identifier* as physical concepts because their daily duty on direct image system encourages them to do so.

Whether one questions the legitimacy/correctness of primary key concept in a theoretical standpoint seems to me as being a separate issue. If majority of SQL practionners were actually asking the same questions that Jan, David Cressey asked, examples such as the one posted probably would not exist...

> 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.
Agreed. I do not think there is much to say on that..This is more of a terminology issue....I do not think however that the concept of *unique identifer* is outside of RM and even less that it could be a physical concept...

Regards... Received on Sun Jul 15 2007 - 14:44:26 CEST

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