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

From: David Cressey <cressey73_at_verizon.net>
Date: Tue, 31 Jul 2007 11:38:07 GMT
Message-ID: <zaFri.3043\$6f4.2362_at_trndny01>

"Brian Selzer" <brian_at_selzer-software.com> wrote in message news:kREri.12113\$eY.9117_at_newssvr13.news.prodigy.net...
>
> "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.
>

Let me be a little more precise: a collection of numbers can be summed, even if each of those numbers represents a temperature. The resulting sum is not meaningful, although if one were to then divide the sum by the count, the result would be the average temperature, which is meaningful.

This distinction between "summing temperatures" and "summing numbers that represent temperatures" is at the heart of the running debate between you and me (and possibly between you and the newsgroup consensus) concerning the update of keys.

Methematically, one can surely update (some of) the key values of (some of) the rows in a relation, yielding a new relation. The question is what this represents, in the real world. One case I want to discard is "error correction". The relationship between a database that is in error and the subject matter it purports to represent is generally confused.

> > 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.

If the before and after values of the update are both correct, how can you update only some of the keys without breaking one or more of the FDs the underly the relation?

> (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.

If there are multiple keys in a relation, they are bound together by FDs. How can one change one key without either changing all the other keys or breaking an FD? Received on Tue Jul 31 2007 - 13:38:07 CEST

Original text of this message