Re: A pk is *both* a physical and a logical object.
Date: Mon, 06 Aug 2007 06:51:52 GMT
"paul c" <toledobythesea_at_oohay.ac> wrote in message
> Brian Selzer wrote:
>> "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.
> 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. What is the point of a database theory newsgroup if it is not to discuss how to persist and manipulate information? What is information anyway? Facts, right? Facts about what? Individuals: without individuals, there can be no information. Without an understanding of how individuals combine to form ground facts, how can one possibly understand the representation of those facts in a database.
The applications that use a database are incidental. There can be many applications that use the same database, or there can be one database per application. Often it becomes necessary to combine databases. Don't you think it's worthwhile to understand how "constructs" in one database can relate to "constructs" in another?
And by the way, this newsgroup has been pretty dead lately.
Received on Mon Aug 06 2007 - 08:51:52 CEST