Re: A pk is *both* a physical and a logical object.
Date: Sun, 05 Aug 2007 14:26:50 GMT
"JOG" <jog_at_cs.nott.ac.uk> wrote in message
> 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. Received on Sun Aug 05 2007 - 16:26:50 CEST