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)

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.

Query example 4: contents of a bag

SELECT ?x, ?y
WHERE (<http://never/bag>, ?x, ?y)
AND ! ( ?x eq <rsyn:type> && ?y eq <rsyn:Bag>)
USING rsyn FOR <http://www.w3.org/1999/02/22-rdf-syntax-ns#>

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

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: