Re: Separate PK in Jxn Tbl?

From: Roy Hann <specially_at_processed.almost.meat>
Date: Mon, 28 Jan 2008 20:49:57 -0000
Message-ID: <>

"Brian Selzer" <> wrote in message news:odonj.910$
> I don't think it's productive to blame it on the users. The problem
> you're referring to is due to the nature of keys, and misunderstandings on
> the part of the database designer as to what constitutes a key. All that
> is required for a key to be a key is that in every possible database
> instance, a projection over the attributes in the key for a relation has
> the same cardinality as the relation. This does /NOT/ mean that a
> particular combination of values /always/ identifies the same individual
> in the Universe of Discourse, but only in the picture of the Universe that
> is a database instance. In other words, a particular combination of
> values may not /necessarily/ identify an individual, but rather may only
> /contingently/ identify an individual. This is the nature of keys: either
> the values for a key are permanent identifiers, or they're not. It has
> nothing to do with how well keys are managed. The values for a key may be
> managed perfectly, yet still not be permanent identifiers--the position of
> something in a list of things comes to mind.

No, the position number identifies the same position, for all time. The facts about the current occupant of the position may change willy-nilly.

Roy Received on Mon Jan 28 2008 - 21:49:57 CET

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