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...)
Barry
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
