A Programmer Introduction to RDQL(from jena web site)

RDQL is a query language for RDF in Jena models.  The idea is to provide a data-oriented query model so that there is a more declarative approach to complement the fine-grained, procedural Jena API.

It is "data-oriented" in that it only queries the information held in the models; there is no inference being done.  Of course, the Jena model may be 'smart' in that it provides the impression that certain triples exist by creating them on-demand.  However, the RDQL system does not do anything other than take the description of what the application wants, in the form of a query, and returns that information, in the form of a set of bindings.

RDQL is an implementation of the SquishQL RDF query language, which itself is derived from rdfDB.  This class of query languages regards RDF as triple data, without schema or ontology information unless explicitly included in the RDF source.

RDF provides a graph with directed edges - the nodes are resources or literals.  RDQL provides a way of specifying a graph pattern that is matched against the graph to yield a set of matches.  It returns a list of bindings - each binding is a set of name-value pairs for the values of the variables.  All variables are bound (there is no disjunction in the query).