Re: A pk is *both* a physical and a logical object.
Date: Tue, 17 Jul 2007 21:15:38 GMT
"Roy Hann" <specially_at_processed.almost.meat> wrote in message
> "Brian Selzer" <brian_at_selzer-software.com> wrote in message
>> 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"?
> 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.
Received on Tue Jul 17 2007 - 23:15:38 CEST