I am using SPARQL Query tab in Protege 5 to query an OWL ontology I have been constructing. I succeded in many kinds of queries, but when I use some specific class of my ontology inside the very same queries (that are apparently well formed) they return no results. Following, two of the problematic queries - assuming "Event" as one of the concepts of the ontology (http://www.semanticweb.org/ontologies/2014/5/MyOnto#Event):
PREFIX rdf: <http://www.w3.org/1999/02/22-rdf-syntax-ns#>
PREFIX owl: <http://www.w3.org/2002/07/owl#>
PREFIX xsd: <http://www.w3.org/2001/XMLSchema#>
PREFIX rdfs: <http://www.w3.org/2000/01/rdf-schema#>
PREFIX onto: <http://www.semanticweb.org/ontologies/2014/5/MyOnto#>
SELECT ?a WHERE { ?a rdfs:subClassOf onto:Event }
and, with the same prefixes
SELECT ?a WHERE { ?a rdfs:range onto:Event }
Both return no results. However, if I substitute "onto:Event" for, let's say, ?b, both return a long list of results - inclunding Event as a match for ?b.
Is it something I'm misusing or forgetting (although I've seen this pattern in several links on internet with people claiming to have got results) or is it a limitation of SPARQL or some issue of the Protege tab?
The problem is that, in fact, although the URI of the ontology is:
<http://www.semanticweb.org/ontologies/2014/5/MyOnto#>
in the OWL document, the prefix used before class names is the IRI:
<http://www.semanticweb.org/ontologies/2014/2/untitled-ontology-662#>
Thus, replacing the old onto: by
PREFIX onto: <http://www.semanticweb.org/ontologies/2014/2/untitled-ontology-662#>
solves the issue.
(Thanks to @Csongor from Protégé Project mail list, who found the answer.)
P.S.: It's also worthy to note that it can't be taken for granted that all the terms in the ontology will be <current_ontology_URI#term> - e.g. if one includes some terms in the ontology and then changes ontology URI, these terms will be identified as <previous_ontology_URI#term> and the new ones as <current_ontology_URI#term> (which was exactly the cause of the problem above).