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

"David Cressey" <cressey73_at_verizon.net> wrote in message news:qOjri.5936$Lu4.4030_at_trndny03...
>
> "Brian Selzer" <brian_at_selzer-software.com> wrote in message
> news:44eri.25057$RX.13408_at_newssvr11.news.prodigy.net...
>
>> I disagree totally. Keys can be the target of an update. Propositions
> are
>> not removed or replaced: they are assigned a different truth value. And
> it
>> is the referents of the propositions that bind them.
>
> This is pretty much meaningless. Temperatures can be summed. That
> doesn't
> mean that the result is meaningful. If fact, one sums a collection of
> temperatures as an intermediate step in calculating the average
> temperature.
> While the sum of a collection of temperature has no meaning, the average
> does.
>

I don't think temperatures /can/ be summed: a temperature may be identifiable by its magnitude, but that magnitude isn't all that is a temperature. One does not sum temperatures, one sums the measurement or representation of temperatures' magnitudes.

> So, what does it mean when a key is updated?

What does it mean? Well, that depends:

(1) It depends upon the number of keys defined on the relation schema and whether all or only some of the keys are the target of the update. (2) It depends upon whether each value for the key permanently identifies /the/ individual referenced by the tuple that contains the key value. Emphasis on /the/ because it is possible that not all key values in a tuple refer to the same individual. An update may target a key that is not the subject or the entire subject.
(3) It depends upon whether the key participates in an inclusion dependency and if it does, whether every instance of the key throughout the database is also affected by the update. Received on Tue Jul 31 2007 - 13:15:28 CEST