Re: plural vs singular properties (a proposal)

On Jan 5, 2008, at 4:08 PM, Garret Wilson wrote:
> Frank Manola wrote:
>>
>> I don't think it's as straightforward as you think to take Date's 
>> comments about one notation for relations and apply them to 
>> another (RDF). See below.
>>
> ...
>>
>> Now, while the representation is different, I would claim that the 
>> semantics (the intended meaning) of these are the same. You would 
>> simply have to have agreement with those you're communicating with 
>> as to how to interpret one of these representations as the more 
>> abstract relational interpretation that Date is talking about.
>
> That's an understandable and not unreasonable view of the 
> situation. It seems to have several implications:
>
> 1. The relational model, then, is a rigorously defined data storage 
> framework that may reflect semantics, but the semantics it reflects 
> depend on external agreements by two communicating parties. (i.e. 
> Using the a relation header to mean "predicate" is one 
> interpretation among many.)
Not quite. For example, I doubt that Date would agree that the 
relational model defines a "data storage framework". It's a model at 
roughly the same level as ordinary n-ary logic. And at least some of 
the agreements I was talking about were about how to interpret a more 
concrete notation (reflecting, e.g., how I'd encoded the header 
information) as this more abstract logical (or relational) notation. 
Given a single relational tuple, like subject(#myBook, airplanes), 
the tuple can always be interpreted both as conveying its intended 
interpretation (there is a book named #myBook whose subject is 
airplanes) and information about the tuple itself (the relation name 
of this relation is "subject").
>
> 2. The RDF model is different from the relational model in this 
> regard; it is a rigorously defined data storage framework the 
> semantics of which are unambiguously defined by the framework 
> itself, not by agreement between two communicating parties. (Of 
> course, two communicating parties may add *additional* semantics.)
Rather, RDF is a version of the relational model where, by 
restricting some of the variability of the full n-ary relational 
model, some of the semantics that require specific agreements among 
the parties are fixed in advance (the pre-agreements are built into 
the restrictions you've agreed to when using RDF).
>
> (Surely one must say that <rdf:Description rdf:about="#mybook" 
> dc:title="My Book"/> has only one interpretation, namely, that some 
> resource identified by <#mybook> has a dc:title of "My Book". That 
> the semantics of dc:title is supplied outside the framework is 
> beside the point here---that applies equally to Date's example and 
> to mine.)
Yes.
>
> 3. The semantics of the relational model when "interpreted in an 
> obvious way" by Date in his wine example is an interpretation 
> incompatible with and therefore unsuitable for representing RDF 
> because it does not allow each predicate to be duplicated in the 
> relation header.
I don't think it's a matter of the interpretation being 
incompatible. The relational model simply requires that when you 
have a situation in which it appears a predicate must be repeated (or 
a single predicate must have multiple values), you must define a 
separate relation. RDF simply assumes ahead of time that all 
predicates may potentially be like this, and restricts itself to 
binary relations.
>
>>> Whew---did I get all that correct?
>>
>> I don't think so! As I said earlier, I think the "equivalent 
>> relational semantics" involve an interpretation of some explicit 
>> notation for relational tables similar to that involved in 
>> interpreting the RDF triples.
>
> I didn't understand the part about "similar to that involved in 
> interpreting the RDF triples", because as I understand it (see #2 
> above) RDF triples have only one interpretation, that of subject/ 
> predicate/object. (The issue of the actual semantics of a 
> particular predicate is a different issue; with a relation, there 
> are several way to represent a predicate without knowing what it 
> means, but in RDF there is only one way to represent a predicate.)
Yes, there is only one interpretation as subject/predicate/object, 
but of course what those subjects, predicates, and objects represent 
as far as the application domain is concerned is something else 
again. In fact, a simple example of this is the business of how to 
represent n-ary relations in RDF (http://www.w3.org/TR/swbp-n- 
aryRelations/). In those examples, instead of having what might be 
considered a simple property (like "diagnosis"), I have to make 
diagnosis a new class of *subject*, actually a reified relation, with 
separate properties of its own.
>
> Garret

Received on Saturday, 5 January 2008 21:54:16 UTC

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