ARQ Making a Query from Scratch - sparql

Question

I have a problem in building the query from scratch syntactically or in algebra, based on
https://jena.apache.org/documentation/query/manipulating_sparql_using_arq.html
For example I have the below query
SELECT (count(?instance) AS ?count)
WHERE
{ ?instance <http://www.w3.org/1999/02/22-rdf-syntax-ns#type>
<http://data.linkedmdb.org/resource/movie/film> }
(project (?count)
(extend ((?count ?.0))
(group () ((?.0 (count ?instance)))
(bgp (triple ?instance <http://www.w3.org/1999/02/22-rdf-syntax-ns#type> <http://data.linkedmdb.org/resource/movie/film>)))))
Can any one direct me with a sample code of how to build the above query from scratch?
I have tried to build it syntactically but failing to know about how to alias the aggregation above.
If anybody can at least guide me in including aggregation with its aliasing name in projection it will be very great.

Answer 1

I don't typically construct queries through code, since I can just parse a query string, or use a parameterized SPARQL query, but here's a reconstruction of your query using the API. Most of the methods I used here I found by exploring the autocomplete options in Eclipse, and by looking at the Javadoc.
import com.hp.hpl.jena.graph.Node;
import com.hp.hpl.jena.graph.Triple;
import com.hp.hpl.jena.query.Query;
import com.hp.hpl.jena.query.QueryFactory;
import com.hp.hpl.jena.sparql.core.Var;
import com.hp.hpl.jena.sparql.expr.ExprAggregator;
import com.hp.hpl.jena.sparql.expr.ExprVar;
import com.hp.hpl.jena.sparql.expr.aggregate.AggCountVar;
import com.hp.hpl.jena.sparql.syntax.ElementTriplesBlock;
import com.hp.hpl.jena.vocabulary.RDF;
public class QueryBuilding {
public static void main(String[] args) {
// Create the query and make it a SELECT query.
final Query query = QueryFactory.create();
query.setQuerySelectType();
// Set the projection expression.
final ExprVar instance = new ExprVar( "instance" );
query.getProject().add( Var.alloc( "count" ), new ExprAggregator( instance.asVar(), new AggCountVar( instance )));
// Construct the triples pattern and add it.
final ElementTriplesBlock triples = new ElementTriplesBlock();
final Node film = Node.createURI( "http://data.linkedmdb.org/resource/movie/film" );
triples.addTriple( new Triple( instance.getAsNode(), RDF.type.asNode(), film ));
query.setQueryPattern( triples );
// Show the query
System.out.println( query );
}
}
The output (i.e., the printed query) follows. It's the same as your query, modulo some whitespace location and newlines.
SELECT (count(?instance) AS ?count)
WHERE
{ ?instance <http://www.w3.org/1999/02/22-rdf-syntax-ns#type> <http://data.linkedmdb.org/resource/movie/film> .}

Answer 2

Although the solution proposed by Joshua was very helpful to me and produces the correct String output, I have found that it contains a problem; the line :
query.getProject().add( Var.alloc( "count" ), new ExprAggregator( instance.asVar(), new AggCountVar( instance )));
Should be replaced by :
query.getProject().add( Var.alloc( "count" ), query.allocAggregate( new AggCountVar( instance ) ));
Otherwise if you execute the query against a Model you will get an exception "NotAVariableException: Node_variable (not a Var) found"