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

From: paul c <toledobythesea_at_oohay.ac>
Date: Tue, 17 Jul 2007 23:36:03 GMT
Message-ID: <Dncni.127379$1i1.54494@pd7urf3no>

Brian Selzer wrote:
> "Roy Hann" <specially_at_processed.almost.meat> wrote in message
> news:7e2dncCYhYD45wHbRVnytAA_at_pipex.net...
>

>>"Brian Selzer" <brian_at_selzer-software.com> wrote in message 
>>news:1Pwmi.39574$Um6.32783_at_newssvr12.news.prodigy.net...
>>[snip]
>>
>>>In addition, the definition of a candidate key does not demand that its 
>>>values rigidly designate individuals in the universe of discourse.
>>
>>I've been waiting for someone else to pick up on this comment, but since 
>>they haven't I'm going to bite.  When you talk about "individuals" here, 
>>do you intend that to be understood as "individual propositions"?
>>

>
> No.
>

>>My layman's understanding is that the value of a candidate key most 
>>definitely *is* a unique identifier (and is irreducible).
>>

>
>
> Indeed. But does the same key value identify the same individual in all
> possible relation values? Clearly this is not the case. Consider a
> relation schema in which the entire heading is the key. Now suppose that a
> user issues a update. The tuple that identified an individual in the
> universe prior to the update is now different from the tuple that identifies
> the same individual after the update. ...

It is different, but I think that is beside the point, which is that after the "update", the user is looking at a different relation. Nothing in the original relation has changed.

p Received on Tue Jul 17 2007 - 18:36:03 CDT