Re: Representing NULL in RDF

Apologies, I'm reading out of order - David and Pat seem to have 
introduced already what OWA mandates here (in absence of a relationship).

Let me just reiterate, then, what I'm trying to say - with an RDBMS hat 
on, I agree that sometimes a null is a 'positive' null, and I too would 
like sometimes to say that in RDF.

(With OWL could I not define a subclass of the relationship domain with 
a zero cardinality constraint on the property and make the 
'positively-null' an instance of that subclass? Apologies if this too 
has been covered...)


On 03/06/13 21:58, Barry Norton wrote:
> On 03/06/13 16:52, Phillip Lord wrote:
>> Value unknown is easy. Just don't say anything.
>> Value not applicable and doesn't exist, given your examples, seem the
>> same to me.
> I don't agree. Under the Open World assumption anything that can later 
> be learned should not affect consistency. A positive null ("there is 
> no such") should lead to a contradiction is someone later asserts such 
> a value/relationship (which doesn't happen with a simple unpopulated 
> relationship from the subject).
> In RDF (which has been my answer before - feel free to contradict me, 
> I'm not an authority, it's just that I've made the proposal before) 
> this seems possible only with Collections. What I mean by this is that 
> with a list-ranged relationship I can specify a value of 'there is no 
> such' (rdf:nil) and someone trying to populate the list later would 
> have to revert that fact to provide such a value. Without a list 
> expected, this does not seem possible (again, to me).
> Barry

Received on Monday, 3 June 2013 21:06:32 UTC