Re: Separate PK in Jxn Tbl?

From: Roy Hann <specially_at_processed.almost.meat>
Date: Tue, 29 Jan 2008 07:37:19 -0000
Message-ID: <>

"Brian Selzer" <> wrote in message news:Uqtnj.1878$
> "Roy Hann" <specially_at_processed.almost.meat> wrote in message
>> 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.
> I don't think so. In the domain of positions, a position number
> identifies the same position, for all time: that is the nature of a
> domain. But whenever a position number appears as a key in a relation, it
> identifies an occupant, even though the occupant identified may not be the
> same occupant at different database instances: this is the nature of a
> key.

A key as you defined it earlier is sufficient to provide only addressability, not identification. They are not the same thing (although anything that provides identification would also be a key). This is a basic part of the concept of functional dependency.

Roy Received on Tue Jan 29 2008 - 08:37:19 CET

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