Re: bc inferencing on optional query patterns

Hi Steve,

Steve Harris writes:
 > 
 > I think it actually becomes:
 > 
 > select ?obj, ?objlabel
 > where    (<http://example.com/foo> ?prop ?obj)
 > OPTIONAL (?obj ?tmp ?objlabel)
 >          (?tmp rdfs:subPropertyOf rdfs:label)
 > 
 > an optional subgraph, c.f. http://www.w3.org/2001/sw/DataAccess/tests/
 > but that still breaks the rule of having variables that can only be bound
 > in the optional part, but can, I think be expanded to something like:
 > [using SQL+RDF and a subset of 3stores schema]
 > 
 > SELECT t1.object, t2.object
 > FROM triples t1
 > LEFT JOIN triples t2 ON
 >       (t2.subject = t1.object)
 > LEFT JOIN triples t3 ON
 >       (t3.subject = t2.predicate AND
 >        t3.predicate = <rdfs:subPropertyOf> AND
 >        t3.object = <rdfs:label>)
 > WHERE t1.subject = <http://example.com/foo>
 >

I'm having problems getting this to work. I think the problem is that
t2 can match any row as long as t2.subject = t1.object, since the bit
that constrains the rest of the pattern is itself a left join.

Does that make sense?

Many thanks,

Phil