FalaG FalaG - 2 years ago 79
Java Question

SPARQL Query returns nothing from a specific RDF graph

After creating an RDF graph using RDFLib in python to apply a sensor ontology ( I used for that a sensor ontology, used also namespace and Bnode which is a blank node representing a resource for which a URI or literal is not given). I tried to query the data in java using sparql therefore I had to store the graph using Jena TDB first then I executed a really simple query which is :

String qs1 = "SELECT * {?s ?p ?o} LIMIT 10" ;

and I used

String source = "/path/graph.rdf";
FileManager.get().readModel( tdb, source);
dataset.begin(ReadWrite.READ) ;
String qs1 = "SELECT * {?s ?o ?p } " ;

try(QueryExecution qExec = QueryExecutionFactory.create(qs1, dataset)) {
ResultSet rs = qExec.execSelect() ;
ResultSetFormatter.outputAsJSON(rs) ;

to execute the query and observe the data in json format.
The problem I m facing is that it returns nothing!
this is the output:

"head": {
"vars": [ "s" , "o" , "p" ]
} ,
"results": {
"bindings": [


I made a simple code to verify if the data are stored :

StmtIterator iter = tdb.listStatements();
// print out the predicate, subject and object of each statement
while (iter.hasNext()) {
Statement stmt = iter.nextStatement(); // get next statement
Resource subject = stmt.getSubject(); // get the subject
Property predicate = stmt.getPredicate(); // get the predicate
RDFNode object = stmt.getObject(); // get the object

System.out.print(" " + predicate.toString() + " ");
if (object instanceof Resource) {
} else {
// object is a literal
System.out.print(" \"" + object.toString() + "\"");

System.out.println(" .");

and indeed they are stored on the TDB database. This is some of the output, which include a bizarres representation of the Bnode and according to some articles its the way the TDB react with Bnode which makes it looks like that.

6f98bd70:1543430b66e:-7fc3 http://www.loa-cnr.it/ontologies/DUL.owl#hasDataValue "37^^file:///data/rbe/workspace/openmtc-python/openmtc-gevent/xsd.float" .
-6f98bd70:1543430b66e:-7fc2 http://purl.oclc.org/NET/UNIS/fiware/iot-lite#hasunit http://purl.oclc.org/NET/ssnx/qu/unit#hPa .
-6f98bd70:1543430b66e:-7fc2 http://www.loa-cnr.it/ontologies/DUL.owl#hasDataValue "996.94^^file:///data/rbe/workspace/openmtc-python/openmtc-gevent/xsd.float" .
-6f98bd70:1543430b66e:-7fc1 http://purl.oclc.org/NET/UNIS/fiware/iot-lite#hasunit http://purl.oclc.org/NET/ssnx/qu/unit# .
-6f98bd70:1543430b66e:-7fc1 http://www.loa-cnr.it/ontologies/DUL.owl#hasDataValue "OK^^file:///data/rbe/workspace/openmtc-python/openmtc-gevent/xsd.float" .
-6f98bd70:1543430b66e:-7fc0 http://purl.oclc.org/NET/UNIS/fiware/iot-lite#hasunit http://purl.oclc.org/NET/ssnx/qu/unit#C .
-6f98bd70:1543430b66e:-7fc0 http://www.loa-cnr.it/ontologies/DUL.owl#hasDataValue "24.2^^file:///data/rbe/workspace/openmtc-python/openmtc-gevent/xsd.float" .

I also tried another graph which uses the friend of a friend ontology and it works fine and correctly.
is it possible that the Bnode is causing this issue ?

Answer Source

try: SELECT * { { ?s ?p ?o } UNION { GRAPH ?g { ?s ?p ?o } } }

Your comments suggest the data is in a named graph but you asked the query of the unnamed/default graph only. The query suggested finds everything, anywhere in the dataset.

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