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

From: Cimode <>
Date: Fri, 13 Jul 2007 05:35:55 -0700
Message-ID: <>

> 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.
I did not understand that and, frankly, I don't care much.

> 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,
Interesting and why would you say that? Forget about the pointless SQL school of thought or mindset or chocolate argument. I am curious as to why you consider that the concept of primary is outside of RM.

> without it. I wanted either confirmation or refutation of that perception
> from this group.
See the answer to Roy Hann.

> It seems to me that, given that as a starting point, assuming you agree,
> you could either endorse or disparage the use of primary key without the
> necessity of endorsing or disparaging the entire community of SQL
> practitioners.
I have already answered this question. Whether there would or would not be a SQL school of thought has abolutely no interest to me.

> Specifically, does Tutorial D have the concept of "primary key" in it?
To my knowledge: no it primarly favors *candidate key* terminology. Check for more info on that...

> If so, how does it relate to relational theory?

Before it became a coined term, primary key was intially used as substitute for unique identifier in Codd's first paper (if I remember showing an example on his initial paper). As direct image systems took over, it became a quasi synonym for index. I believe this term has lost its meaning. While it designates something precise it has been so misused that I perfectly understand the motivation of some of simply dumping it and leaving it to SQL community.

> If not, how does it address
> the same problems that lead SQL practitioners to lean on the concept of
> primary key?
Is there a concept of primary key in SQL? SQL practitionners (including myself) spending more time worrying about physical pointers then truly dealing with primary keys in a logical perspective.

Regards... Received on Fri Jul 13 2007 - 14:35:55 CEST

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