chapter  6
10 Pages

Query Languages for SWL-based Knowledge Bases

ByUmberto Straccia

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SPARQL [380, 381] is a query language and a protocol for data that is stored natively as RDF or viewed as RDF via middleware. The main mechanism for computing query results in SPARQL is subgraph matching: RDF triples in both the queried RDF data and the query pattern are interpreted as nodes and edges of directed graphs, and the resulting query graph is matched to the data graph using variables as wild cards. Various W3C standards, including RDF and OWL, provide semantic interpretations for RDF graphs that allow additional RDF statements to be inferred from explicitly given assertions. Many applications that rely on these semantics require a query language such as SPARQL, but in order to use SPARQL, basic graph pattern matching has to be defined using semantic entailment relations instead of explicitly given graph structures. There are different possible ways of defining a basic graph pattern matching extension for an entailment relation. [379] specifies one such way for a range of standard Semantic Web entailment relations. Such extensions of the SPARQL semantics are called entailment regimes. An en-tailment regime defines not only which entailment relation is used, but also which queries and graphs are well-formed for the regime, how the entailment is used (since there are potentially different meaningful ways to use the same entailment relation), or what kinds of errors can arise.