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.

Then what the heck *are* you talking about? What is an "individual" (in the context of a database)?

>> 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.

Under the closed world hypothesis, the only sensible reason to do the update you describe to a row in a relation in which the entire header is the key would be to retract a falsehood. I have no interest in falsehoods and I don't see how they are related to "individuals" (whatever they are). What am I not getting?

Roy Received on Wed Jul 18 2007 - 00:47:59 CEST