Re: Separate PK in Jxn Tbl?
Date: Mon, 28 Jan 2008 20:49:57 -0000
"Brian Selzer" <brian_at_selzer-software.com> wrote in message
> 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