Re: General semantics

On 26 mei, 03:46, "Mr. Scott" <do_not_re..._at_noone.com> wrote:
> "Erwin" <e.sm..._at_myonline.be> wrote in message
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> news:4df4d884-e6bb-427e-b97b-96647f171a11_at_m33g2000vbi.googlegroups.com...
> On 25 mei, 23:04, "Mr. Scott" <do_not_re..._at_noone.com> wrote:
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> > "paul c" <toledobythe..._at_oohay.ac> wrote in message
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> >news:YodKn.4587$Z6.2983_at_edtnps82...
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> A relation is what has been recorded, so there is nothing wrong with
> referring to what it records.

Interpreting the meaning of a relation can only be done when some predicate is given to do that interpreting. Predicates are tied to relation _variables_, not to relations. TABLE_DEE appearing as the value for relvar THE_SHOP_IS_OPEN means something different than TABLE_DEE appearing as the value for relvar THE_ALARM_IS_SET.

Saying that relations have meaning irrespective of some predicate coming from some relvar, is a plain and simple admission of the fact that you simply aren't getting it.

> > Both indicate that there are three cans of dog food, but does the second
> > indicate that there is no such thing as a can of cat food, or is it
> > synonymous with the first?

It depends on the predicate of the relvar that these relations might appear for.

If the predicate of that relvar is "The shop sells <x>, and the current quantity available/in stock is <y>", then the absence of any tuple for "cans of cat food" implies that the shop simply does not ever sell such a thing. Presence of a tuple for "cans of cat food" with quantity zero, would imply that the shop does sell cans of cat food, but that it currently cannot do so because the shop hasn't got any in stock. Both meanings are quite different.

If the predicate is "The quantity currently available for product <x> is <y>", then it is indeed true that propositions including the quantity zero can just as well not be made. My understanding is that you are trying to turn this into a deficiency of the relational model. It is not. It is just a property of the number zero, and it is just the logical meaning of any quantity being zero.

Same can be said of the kinds of predicate/proposition that I think Paul was referring to : it is true that it is not true that my name is John Doe". Artificially fabricated arguments involving the CWA applied to such predicates are not a valid criticism of the relational model.

> It doesn't matter what the predicate is because the propositions in question
> differ in form:

I have demonstrated sufficiently why and how you are wrong.

> > Under the closed world assumption, the
> > proposition that is the result of substituting the values "can of cat
> > food"
> > and 0 for the variables in the predicate for the second relation is
> > supposed
> > to be false because the tuple doesn't appear in the relation!

> But there is the contradiction. It doesn't appear in the second relation but
> the absence of it logically implies it: that doesn't make any sense.

There is no contradiction. At best, in a certain particular case (which involves the _assumption_ that you have silently made about what the predicate is), there is only semantic equivalence between making an explicit statement about some quantity being zero, and making no statement at all. Once again, that is not a deficiency of the relational model, it isn't even a deficiency of any data model what so ever, it is just a peculiar property of the number zero when it is used to express quantities. Accounting software already rejected bookings with a zero amount long before the concept of databases (let alone relational ones) was invented.

As I cannot explain any better, I won't bother any further. Received on Wed May 26 2010 - 11:23:45 CEST