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

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. 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