Examples

Query example 1: Retrieve the value of a known property of a known resource

SELECT ?x
WHERE  (<http://somewhere/res1>, <http://somewhere/pred1>, ?x)

Query example 2: constraints

SELECT ?a, ?b
WHERE  (?a, <http://somewhere/pred1>, ?b)
AND    ?b < 5

Query example 3: paths in the graph

SELECT ?a, ?b
WHERE (?a, <http://somewhere/pred1>, ?c) ,
      (?c, <http://somewhere/pred2>, ?b)

Query example 4: contents of a bag

SELECT ?x, ?y
WHERE (<http://never/bag>, ?x, ?y)
AND ! ( ?x eq rdf:type && ?y eq rdf:Bag)

Query example 5: identifying the RDF model in the query

SELECT ?a, ?b
FROM   <http://somewhere/model1.rdf>
WHERE  (?a, <http://somewhere/pred1>, ?b)
AND    ?b < 5

Usage

There are two ways to use RDQL : from the command line and in Java programs.

Command Line

The main class is jena.rdfquery:

java -cp ... jena.rdfquery [--data modelFile] [--query File | queryString]

With no arguments, it prints a usage message.

Java

Example code fragment 1 (basic template)

Query query = new Query(queryString) ;
// Need to set the source if the query does not.
// This provided this override any query named source.
query.setSource(model);
QueryExecution qe = new QueryEngine(query) ;
QueryResults results = qe.exec() ;
for ( Iterator iter = results ; iter.hasNext() ; )
{
    ResultBinding res = (ResultBinding)iter.next() ;
    // Return from get is null if not found
    Object obj = res.get("x") ;
    // obj will be a Jena object: resource, property or RDFNode.
}
results.close() ;

The ResultBinding class allows an alternative form which can be used to get quoted strings:

Value v = env.getValue("x") ;
String s = (v == null) ? "<null>" : v.asQuotedString() ;

Example code fragment 2 (using the results formatter)

Query query = new Query(queryString) ;
query.setSource(model);
QueryExecution qe = new QueryEngine(query) ;
QueryResults results = qe.exec() ;
QueryResultsFormatter fmt = new QueryResultsFormatter(results) ;
PrintWriter pw = new PrintWriter(System.out) ;
fmt.printAll(pw, " | ") ;
pw.flush() ;
fmt.close() ;	
results.close() ;

The command line usage is a wrapper around the Java interface.

The source for the query would be overidden by the Java call query.setSource, or from the command line using --data URI.

Relative URIs are interpreted as relative filenames.

The syntax of a source, if not given elsewhere, is detemined bythe extension on the URI.  Currently, the type recognized are: