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

On Aug 6, 10:31 am, Jan Hidders <hidd..._at_gmail.com> wrote:
> On 6 aug, 08:51, "Brian Selzer" <br..._at_selzer-software.com> wrote:
>
>
>
> > "paul c" <toledobythe..._at_oohay.ac> wrote in message
>
> >news:yfvti.33584$fJ5.19850_at_pd7urf1no...
>
> > > Brian Selzer wrote:
> > >> "JOG" <j..._at_cs.nott.ac.uk> wrote in message
> > >>news:1185445415.561100.98380_at_o61g2000hsh.googlegroups.com...
>
> > >>>Just as another example of what i'm on about with this construct
> > >>>m'larkey: Imagine the library has two copies of "harry potter and the
> > >>>deathly hallows". Are they the same book?
>
> > >>>1) If your construct is the one that uses the barcode on the sleeve as
> > >>>an identifier, then no, different books.
> > >>>2) If your construct is the one that uses the ISBN number as an
> > >>>identifier, then yes, same book.
>
> > >>>There's no correct answer, and which you pick just depends on the
> > >>>application. A Loans database could use Barcodes; A library listings
> > >>>database could use ISBN.
>
> > >> A very thought-provoking example. Are they the same book? From the
> > >> information given, no, they're not the same book. They are two different
> > >> physical manifestations of the same abstract individual. Abstract
> > >> individuals are incomplete in the sense that they cannot exist apart from
> > >> their physical manifestations, for to exist is to be spatiotemporally
> > >> located. As a consequence, the identity relation fails just in case
> > >> there are no physical manifestations; therefore, it must be assumed that
> > >> there exist physical manifestations. So if each tuple in a relation
> > >> describes a specific abstract individual, then that relation must be a
> > >> projection of another--even if it isn't defined in the schema. Since the
> > >> abstract individual exemplifies all of its physical manifestations and
> > >> cannot exist apart from those physical manifestations, the existence of a
> > >> tuple in a relation that uses ISBNs as key values implies the existence
> > >> of at least one tuple in a relation that uses barcodes as key
> > >> values--even if the barcode relation is not defined in the schema. If at
> > >> some point in the future the loans and library listings databases were
> > >> combined, there would clearly be a cyclical relationship between the set
> > >> of abstract individuals denoted by ISBNs and the set of concrete
> > >> individuals denoted by barcodes.
>
> > > JOG, please correct me if I've got you wrong, but I believe this is trying
> > > to answer a quite different question than what you asked, maybe several
> > > others, in a most futile way to boot, whereas I think you meant "what
> > > constitutes/how do I represent, a book for the lending app and what
> > > constitutes/how do I represent, a book for the inventory app?". I also
> > > read your post as meaning "there is no one correct answer for all apps".
>
> > > I'm guessing the reason that nobody else has continued this thread is that
> > > they are as mystified as I am by the latest mumbo-jumbo. What possible
> > > usefulness does it have other than letting one know that one's usenet feed
> > > is operating?
>
> > It's unfortunate that you can't see the usefulness of a discussion about the
> > relationship between abstract individuals and their concrete physical
> > manifestations--especially in this forum.
>
> I can. Interestingly enough such discussions are often easier in terms
> of ER-like models than the Relational model.

You seem to be saying that such discussions about conceptual modeling are easier in terms of conceptual models rather than logical ones. Is this not a bit self-evident?

> It also usually leads to
> the (re)discovery of the plays_role_of / is_concrete_instance_of
> relationship which is akin to the ISA relationship but without
> inheritance of the keys.
>
> -- Jan Hidders
Received on Mon Aug 13 2007 - 02:37:19 CEST