var query = content_manager.query_manager.create_query_select ("foaf:Person");
query.set_constraint (new SQLQueryConstraint(
    new QueryProperty ("http://xmlns.com/foaf/0.1/currentProject", null),
    "=",
    QueryValue.create_with_value ("http://www.midgard-project.org/"),
    null
));

Generates:

SELECT 
	t1.mgd_id AS id, 
	t1.mgd_guid AS guid, 
	t1.property_value AS value, 
	t1.property_literal AS literal, 
	t1.property_name AS property, 
	t1.object_guid AS objectguid, 
	t1.class_name AS classname, 
	t1.identifier AS identifier
FROM rdf_triple_object AS t1
	LEFT JOIN rdf_triple_object AS t2 ON (t1.object_guid = t2.object_guid)
WHERE ((t1.class_name = 'http://xmlns.com/foaf/0.1/Person') AND (t2.property_name = 'foaf:currentProject') AND ((t2.property_literal = 'http://www.midgard-project.org/') OR (t2.property_value = 'http://www.midgard-project.org/'))) AND 1=1 (0.4453)