RDFLog: It’s like Datalog for RDF

RDF data is set apart from relational or XML data by its support of rich existential information in the form of blank nodes. Where in SQL databases null values are scoped over a single tuple, blank nodes in RDF can span over any number of statements and thus can be seen as existentially quantified variables. Blank node querying is considered in most RDF query languages, but blank node construction, i.e., the introduction of new blank nodes has been mostly ignored (e.g., in Triple) or treated in a very limited form (e.g., in SPARQL). In this paper, we classify three kinds of blank nodes in RDF query languages and introduce the recursive, rule-based RDF query language RDFLog. RDFLog is the first RDF query language with full arbitrary quantifier alternation: blank nodes may occur in the scope of all, some, or none of the universal variables of a rule. RDFLog is also aware of important RDF features such as the distinction between blank nodes, literals and URIs or the RDFS vocabulary.