Re: plural vs singular properties (a proposal)


Andrew Newman wrote:
> Predicates need not be repeated in order to store multiple values -
> the idea of multiple values is rather arbitrary.

But do you agree that, if we consider relation attributes to model RDF 
predicates, that relations do not allow predicates to be repeated?

RDF, on the other hand, allows predicates to be repeated. That contrast 
is what I was trying to communicate, and about which I was trying to 
learn if there is any reason to be alarmed.

> Attributes can also be of type relations (Chapter 6 of Intro to
> Database Systems and Chapter 12.6 has some notes on it).
> In your example you want to store to subjects for a book.  The
> attribute type then becomes a set of strings (or whatever type your
> want to choose for subjects of books).

Yes, I understand that. Date is quite happy with his revelation 
(apparently he didn't believe this was possible in earlier editions of 
the book) that an attribute can be of a relation type and its value can 
itself be a relation. But that doesn't mean I'm putting multiple values 
in a tuple for that column---I'm putting one value, which is itself a 

Similarly, if I want multiple dc:subject predicates, I can make the 
value of type rdf:List. That doesn't mean I'm giving the predicate 
dc:subject multiple values---it just means that its single value holds 
multiple resources.

But (unlike the relational model, I think) in RDF I also have the option 
to repeat the predicate if I want another dc:subject value! Isn't this 
in direct contrast to the relational model? I can't just put "repeat the 
predicate" to add another value on a relation, can I!

Bijan (both on this list and off-line) tells me that I can just add 
another tuple in the relation with the additional dc:subject value (I 
guess Frank said that as well), and that this is semantically equivalent 
to "repeating the predicate" in RDF (I hope I didn't misstate you, Bijan).

And it is at this point my expertise in the relational model breaks 
down; as I told Bijan, I think the best thing for to do at this point is 
to suspend my participation in the discussion while I dive back into 
Date and make my relational foundation a little more solid. So I'll try 
to not say much more here for a bit, until after I've studied more.

(Yes, Frank, my discussion on nulls is on hold as well; your suggestion, 
which you may now rue ;) , that I study the relational model concerning 
nulls is what brought me to this question in the first place.)



Received on Sunday, 6 January 2008 04:10:20 UTC