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_at_pd7urf3no>
>
> No.
>
>
>
> 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. ...
Date: Tue, 17 Jul 2007 23:36:03 GMT
Message-ID: <Dncni.127379$1i1.54494_at_pd7urf3no>
Brian Selzer wrote:
> "Roy Hann" <specially_at_processed.almost.meat> wrote in message
> news:7e2dncCYhYD45wHbRVnytAA_at_pipex.net...
>
>>"Brian Selzer" <brian_at_selzer-software.com> wrote in message >>news:1Pwmi.39574$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 Wed Jul 18 2007 - 01:36:03 CEST