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

On Aug 6, 2:58 am, paul c <toledobythe..._at_oohay.ac> wrote:
> 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".

Yes, that is exactly what I intended.

My fear is that ERM and OO encourage the notion of thinking in terms of fixed objects, as opposed to the flexible overlapping concepts that we actually use. Sure, its fine for those in the know to use them wisely, simply as a means to an end, but it is my experience in education is that they can be really detrimental - and that ultimately results in more people in industry producing more incorrect schemas and brittle software designs.

>
> 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?
>
> p
Received on Mon Aug 13 2007 - 02:29:07 CEST