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Re: plural vs singular properties (a proposal)

From: Frank Manola <fmanola@acm.org>
Date: Sat, 5 Jan 2008 16:54:00 -0500
Message-Id: <AE09E08D-DE8C-4195-8092-5CB66FE4C468@acm.org>
Cc: SWIG <semantic-web@w3.org>
To: Garret Wilson <garret@globalmentor.com>

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.)


> 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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