ARQ - Construct Quad

The current W3C recommendation of SPARQL 1.1 supports the CONSTRUCT query form, which returns a single RDF graph specified by a graph template. The result is an RDF graph formed by taking each query solution in the solution sequence, substituting for the variables in the graph template, and combining the triples into a single RDF graph by set union. However, it does not directly generate quads or RDF datasets.
In order to eliminate this limitation, Jena ARQ extends the grammar of the CONSTRUCT query form and provides the according components, which brings more conveniences for the users manipulating RDF datasets with SPARQL.

This feature was added in Jena 3.0.1.

Query Syntax

A CONSTRUCT template of the SPARQL 1.1 query String is Turtle format with possible variables. The syntax for this extension follows that style in ARQ, using TriG plus variables. Just like SPARQL 1.1, there're 2 forms for ARQ Contruct Quad query:

Complete Form

CONSTRUCT {
    # Named graph
    GRAPH :g { ?s :p ?o }
    # Default graph
    { ?s :p ?o }
    # Default graph
    :s ?p :o
} WHERE { 
    # SPARQL 1.1 WHERE Clause
    ... 
}

The default graphs and the named graphs can be constructed within the CONSTRUCT clause in the above way. Note that, for constructing the named graph, the token of GRAPH can be optional. The brackets of the triples to be constructed in the default graph can also be optional.

Short Form

CONSTRUCT WHERE { 
    # Basic dataset pattern (only the default graph and the named graphs)
    ... 
}

A short form is provided for the case where the template and the pattern are the same and the pattern is just a basic dataset pattern (no FILTERs and no complex graph patterns are allowed in the short form). The keyword WHERE is required in the short form.

Grammar

The normative definition of the syntax grammar of the query string is defined in this table:

Rule Expression
ConstructQuery ::= 'CONSTRUCT' ( ConstructTemplate DatasetClause* WhereClause SolutionModifier | DatasetClause* 'WHERE' '{' ConstructQuads '}' SolutionModifier )
ConstructTemplate ::= '{' ConstructQuads '}'
ConstructQuads ::= TriplesTemplate? ( ConstructQuadsNotTriples '.'? TriplesTemplate? )*
ConstructQuadsNotTriples ::= ( 'GRAPH' VarOrBlankNodeIri )? '{' TriplesTemplate? '}'
TriplesTemplate ::= TriplesSameSubject ( '.' TriplesTemplate? )?

DatasetClause, WhereClause, SolutionModifier, TriplesTemplate, VarOrIri, TriplesSameSubject are as for the SPARQL 1.1 Grammar

Programming API

ARQ provides 2 additional methods in QueryExecution for Construct Quad.

Iterator<Quad> QueryExecution.execConstructQuads() // allow duplication
Dataset QueryExecution.execConstructDataset() // no duplication

One difference of the 2 methods is: The method of execConstructQuads() returns an Iterator of Quad, allowing duplication. But execConstructDataset() constructs the desired Dataset object with only unique Quads.

In order to use these methods, it's required to swtich on the query syntax of ARQ beforehead, when creating the Query object:

Query query = QueryFactory.create(queryString, Syntax.syntaxARQ);

If the query is supposed to construct only triples, not quads, the triples will be constructed in the default graph. For example:

String queryString = "CONSTRUCT { ?s ?p ?o } WHERE ... "
...
// The graph node of the quads are the default graph (ARQ uses <urn:x-arq:DefaultGraphNode>).
Iterator<Quad> quads = qexec.execConstructQuads();

If the query string stands for constructing quads while the method of exeConstructTriples() are called, it returns only the triples in the default graph of the CONSTRUCT query template. It's called a "projection" on the default graph. For instance:

String queryString = "CONSTRUCT { ?s ?p ?o . GRAPH ?g1 { ?s1 ?p1 ?o1 } } WHERE ..."
...
// The part of "GRAPH ?g1 { ?s1 ?p1 ?o1 }" will be ignored. Only "?s ?p ?o" in the default graph will be returned.
Iterator<Triple> triples = qexec.exeConstructTriples();

More examples can be found at ExampleConstructQuads.java under jena-arq/src-examples

Fuseki Support

Jena Fuseki is also empowed with Construct Quad query as a built-in function. No more addtional configuration is required to switch it on. Because QueryEngineHTTP is just an implementation of QueryExcecution, there's no much difference for the client users to manipulate the programming API described in the previous sections, e.g.

String queryString = " CONSTRUCT { GRAPH <http://example/ns#g1> {?s ?p ?o} } WHERE {?s ?p ?o}" ;
Query query = QueryFactory.create(queryString, Syntax.syntaxARQ);
try ( QueryExecution qExec = QueryExecutionFactory.sparqlService(serviceQuery, query) ) { // serviceQuery is the URL of the remote service
    Iterator<Quad> result = qExec.execConstructQuads();
    ...
}
...

ARQ documentation index