Re: On Formal IS-A definition

Erwin wrote:
> On 7 mei, 19:55, paul c <toledobythe..._at_oohay.ac> wrote:
>> Erwin wrote:

...
>>>> Regarding another aspect of the Information Principle, I'd like to ask
>>>> whether an SuppliersParts relation such as:
>>>> S# P#
>>>> S1 P1
>>>> S1 P2
>>>> S2 P3
>>>> has the same information as one with a domain PS# that is a set of part
>>>> numbers:
>>>> S# PS#
>>>> S1 {P1,P2}
>>>> S2 {P3}
>>>> If they have the same information, does that mean they are equivalent,
>>>> ie., each implies the other? If so, is there a practical need for a
>>>> relational equality operator (as opposed to equality between domain values)?
>>> Whether those designs have(/guarantee) the same information depends on
>>> the constraints. You don't mention those.
>>> In your example, there are two cases (wrt the PS# design) :
>>> (a) S# is declared to be a key
>>> (b) S# is not declared to be a key
>>> If (a), then the designs are equivalent.
>>> If (b), then they are not. For in this case, there must have been an
>>> explicit reason to allow both the tuples {S1 {P1}} and {S1 {P1 P2}} to
>>> appear (otherwise the designer could simply have chosen key S#). A
>>> designer explicitly opting to allow both {S1 {P1}} and {S1 {P1 P2}},
>>> must therefore be assumed to attach a different meaning to those two
>>> tuples, meaning the two propositions implied by those two distinct
>>> tuples are unequal, meaning that there is something to the predicate
>>> of this relvar in which a difference of meaning is caused by the set
>>> value of the RVA attribute.
>>> Can't explain it any better than this. Relvar predicates when RVA's
>>> are involved is pretty much an issue that's still left to be resolved,
>>> as I'm sure you're very well aware.
>> Thank you. Regarding "If (a), then the designs are equivalent", can we
>> also say that (logically) equivalent means "same information"?- Tekst uit oorspronkelijk bericht niet weergeven -
>>
>> - Tekst uit oorspronkelijk bericht weergeven -
>
> I suppose so. Do you see a reason for this being questionable ?

Nothing questionable that I can express at the moment, I was just a little taken aback that the question about two specific values being equivalent might depend on design/constraints but perhaps that is what it comes down to. Maybe it was wrong of me to think they could have equivalent predicates, it's just something that I feel a need to grope every so often, not sure why.

(I didn't have RVA's in mind, more what you might call Set-Valued-Attributes and those certainly do need some basis for interpretation, eg., perhaps a tuple with an empty set of parts would need to be meaningless in the face of the closed-world-assumption. Wouldn't surprise me if Codd considered and rejected this at some early point, maybe that was part of the genesis of his 'normal' form - I wonder why he didn't call it 'elemental' form.) Received on Tue May 11 2010 - 19:02:30 CEST