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

"Brian Selzer" <brian_at_selzer-software.com> wrote in message news:_jani.39744$Um6.23567_at_newssvr12.news.prodigy.net...
>
> "Roy Hann" <specially_at_processed.almost.meat> wrote in message
> news:7e2dncCYhYD45wHbRVnytAA_at_pipex.net...
> > "Brian Selzer" <brian_at_selzer-software.com> wrote in message
> > news:1Pwmi.39574$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. Because the entire heading is the
> key, it is clear that although the key uniquely identifies an individual
and
> is irreducible, it does not rigidly designate that individual. A rigid
> designator identifies the same individual in all possible relation values,
> not just the one that happens to be actual at any given point in time.
>

You can call what the user issued an update, if you care to, but what really happened is that an individual, identified by the before value of the key, was deleted, and a new individual, identified by the after value of the key was inserted. The fact that the old value and the new value used the same storage ("row", if you like) is irrelevant. They are different values, and they designate different individuals. Received on Wed Jul 18 2007 - 01:22:53 CEST