Re: A pk is *both* a physical and a logical object.
From: Bob Badour <bbadour_at_pei.sympatico.ca>
Date: Tue, 14 Aug 2007 13:25:34 -0300
Message-ID: <46c1d740$0$4056$9a566e8b_at_news.aliant.net>
>
>
> Yes, pretty much. Lots of possible constructs. Which of the books are
> the same?
>
> 1) All - all harry potter and the deathly hallows (identifying
> attribue for a "book" - title)
> 2) None - all the copies are different (identifying attribute for a
> "book" - barcode)
> 3) The two paperpack versions (identifying attribute for a "book" -
> isbn)
> 4) All the english versions (identifying attribute for a "book" - its
> content)
> 5) All the english versions without illustrations (identifying
> attribute for a "book" - its text)
> 6) etc, etc...
>
> All are valid answers. No context to the question - no suitable
> answer. Pick the wrong one for the context you need, broken schema.
> Here we are comparing different items, but we could just as easily be
> comparing the things at different points in time. Something is only
> the same entity if /for the context we chose/ its identifying
> attribute is the same - all of its other properties may change, but if
> the identifying attribute changes then it is a different thing as far
> as that context is concerned.
>
> Again let me emphasize that this is all at the conceptual level. But
> it is only when one has that level sorted that one can move down to
> the logical encoding.
Date: Tue, 14 Aug 2007 13:25:34 -0300
Message-ID: <46c1d740$0$4056$9a566e8b_at_news.aliant.net>
JOG wrote:
> On Aug 13, 7:27 pm, "Brian Selzer" <br..._at_selzer-software.com> wrote:
>
>>"JOG" <j..._at_cs.nott.ac.uk> wrote in message >> >>news:1187005635.391467.51520_at_l70g2000hse.googlegroups.com... >> >>>On Aug 13, 6:56 am, "Brian Selzer" <br..._at_selzer-software.com> wrote: >>> >>>>"JOG" <j..._at_cs.nott.ac.uk> wrote in message >> >>>>news:1186967829.283726.289850_at_o61g2000hsh.googlegroups.com... >> >>>>>On Aug 5, 3:26 pm, "Brian Selzer" <br..._at_selzer-software.com> 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. >> >>>>>I'm glad you thought it was an interesting example. I personally see >>>>>no distinction between your "abstract" and "physical manifestations". >>>>>To illustrate this all i'm asking is that you just extend the example >>>>>to use more constructs - maybe I now have five books, the two harry >>>>>potters from before, another that's got illustrations, one translated >>>>>into mandarin and a digital version. We now have an almighty >>>>>conundrum if someone asks us "which of these are the same book". How >>>>>do you split up "physical" and "abstract" now? It would be an absolute >>>>>spaghetti to try to hazard an answer! >> >>>>It is simple. An abstract individual cannot be spatiotemporally located. >>>>The one thing that the five individuals above have in common is the >>>>abstract >>>>individual: they are all physical manifestations of it. Neither the >>>>addition of illustrations, the translation into mandarin nor the encoding >>>>into digital form changes the fact that the abstract individual >>>>exemplifies >>>>each of those five tangible instances. >> >>>Nope, you've missed the point. There are now several possible >>>'abstract' individuals. There are now also about a dozen ways of >>>answering the question "which of these books are the same". Have a >>>look at the different possible answers. >> >>I may be dense, but you're right, I've missed your point. There is only one >>abstract individual that exemplifies all of the concrete instances. There >>may be additional abstract individuals, such as the set of illustrations, or >>the translation. Is that your point?
>
>
> Yes, pretty much. Lots of possible constructs. Which of the books are
> the same?
>
> 1) All - all harry potter and the deathly hallows (identifying
> attribue for a "book" - title)
> 2) None - all the copies are different (identifying attribute for a
> "book" - barcode)
> 3) The two paperpack versions (identifying attribute for a "book" -
> isbn)
> 4) All the english versions (identifying attribute for a "book" - its
> content)
> 5) All the english versions without illustrations (identifying
> attribute for a "book" - its text)
> 6) etc, etc...
>
> All are valid answers. No context to the question - no suitable
> answer. Pick the wrong one for the context you need, broken schema.
> Here we are comparing different items, but we could just as easily be
> comparing the things at different points in time. Something is only
> the same entity if /for the context we chose/ its identifying
> attribute is the same - all of its other properties may change, but if
> the identifying attribute changes then it is a different thing as far
> as that context is concerned.
>
> Again let me emphasize that this is all at the conceptual level. But
> it is only when one has that level sorted that one can move down to
> the logical encoding.
If you are not careful, Marvin Minsky will show up to explain frames. Received on Tue Aug 14 2007 - 18:25:34 CEST