⊟ Remarks

This SPARQL engine parses and evaluates a SPARQL language query against any SemWeb.SelectableSource or SemWeb.QueryableSource. If the queried data source is a SemWeb.QueryableSource, the Query method is used where possible. Otherwise, query execution falls back to calling SemWeb.SelectableSource's Contains and Select methods.

This class is a fork of the SPARQL engine library at http://sparql.sourceforge.net/ by Ryan Levering, version 0.8. The upstream library has been heavily modified, however.

This example runs a SELECT query on a data source and writes the results as XML to standard output.

C# Example

MemoryStore datamodel = new MemoryStore(new RdfXmlReader("rdfdata.rdf")); string sparqlQuery = "PREFIX rdf: <http://www.w3.org/1999/02/22-rdf-syntax-ns#>\n" + "PREFIX foaf: <http://xmlns.com/foaf/0.1/>\n" + "SELECT * WHERE { ?person rdf:type foaf:Agent . \n" + "?person foaf:name ?name . }"; SparqlEngine query = new SparqlEngine(sparqlQuery); query.Run(datamodel, Console.Out).

Although this example sends the results to a stream, the results can be processed programmatically by passing a SemWeb.Query.QueryResultSink instead of a TextWriter.

Something must be said about the handling of FROM, FROM NAMED, and GRAPH clauses in queries. The FROM clause is used to specify the set of graphs that combined make up the default dataset, which is what triples in the query are normally (that is, outside of a GRAPH clause) matched against. FROM clauses are essentially ignored by the SPARQL engine. It is up to the caller to check if any FROM clauses are specified using SparqlEngine.GetDefaultDatasets and passing the right data source to the SPARQL engine as appropriate. Outside of a GRAPH clause, the triples in a query are compared against the data source with the SemWeb.Statement.Meta or SemWeb.SelectFilter.Meta field set to SemWeb.Statement.DefaultMeta, the default value when no meta or provenance information is associated with a statement.

Triples inside of a GRAPH clause with an explicit URI, such as "GRAPH <....uri...> { ...graph pattern... }", are matched against the data source by passing the URI specified in the graph clause as the value of SemWeb.Statement.Meta or as a one-element array in SemWeb.SelectFilter.Meta.

In GRAPH clauses with graph variables, such as "GRAPH ?g { ...graph pattern... }", the potential values of the graph variable can sometimes be inferred from previous statements referencing the graph variable that were already evaluated during the processing of the query. Failing that, the potential values of graph variables are drawn from the set of FROM NAMED graphs, if any were specified. If any potential values for the graph variable are known, they are passed together in the SemWeb.SelectFilter.Meta field in calls to Contains, Select, or (if there is just one such value) Query on the data source, or if there is more than one value then in a call to Query the SemWeb.Statement.Meta field is set to a new SemWeb.Variable and QueryOptions.VariableKnownValues is filled in with the potential values for the variable

In GRAPH clauses with graph variables whose values are not known at all, null is passed as the value of SemWeb.SelectFilter.Meta, or in a call to SemWeb.QueryableSource.Query(SemWeb.Statement[], QueryOptions, QueryResultSink) the SemWeb.Statement.Meta field is set to a new SemWeb.Variable.

Since, for instance, it is impossible to set the Meta field of statements in RDF/XML, it is recommended in order to process GRAPH clauses to implement your own SemWeb.SelectableSource to pass as the data source for queries that will inspect the Meta field and pass off calls to Contains and Select to the correct source graph.

⊟ Requirements

Namespace: SemWeb.Query
Assembly: SemWeb.Sparql (in SemWeb.Sparql.dll)
Assembly Versions: 0.0.0.0

⊟ Members

See Also: Inherited members from Query.

⊟ Public Constructors

	SparqlEngine (System.IO.TextReader)	Initialized the SPARQL query from a TextReader.
	SparqlEngine (string)	Initializes the SPARQL query from a string.

⊟ Public Fields

AllowPersistBNodes

bool . Documentation for this section has not yet been entered.

⊟ Public Properties

	MimeType	string . Gets or sets the preferred MIME type for the output of the query. (Inherited from Query.)
	QueryMeta	SemWeb.Entity . A filter on the Meta value of statements in the target graph considered by the query. (Inherited from Query.)
	ReturnLimit	int . The number of bindings to return. (Inherited from Query.)
	ReturnStart	int . The index of the first binding to return. (Inherited from Query.)
[read-only]	Type	SparqlEngine.QueryType . Gets the type of the query.

⊟ Public Methods

	AddExternalFunction (RdfFunction) Registers an external function that can be used in FILTER clauses.
	Ask (SemWeb.SelectableSource) : bool Executes an ASK query on the given data source and returns the result of the query as a boolean.
	Ask (SemWeb.SelectableSource, System.IO.TextWriter) Executes an ASK query on the given data source and outputs the result of the query in the SPARQL results format to a TextWriter.
	Construct (SemWeb.SelectableSource, SemWeb.StatementSink) Executes a CONSTRUCT query on the given data source and sends the constructed statements to a StatementSink.
	Construct (SemWeb.SelectableSource, System.IO.TextWriter) Executes a CONSTRUCT query on the given data source and outputs the result of the query in the SPARQL results format to a TextWriter.
	Describe (SemWeb.SelectableSource, SemWeb.StatementSink) Executes a DESCRIBE query on the given data source and sends the constructed statements to a StatementSink.
	Describe (SemWeb.SelectableSource, System.IO.TextWriter) Executes a DESCRIBE query on the given data source and outputs the result of the query in the SPARQL results format to a TextWriter.
	GetDefaultDatasets () : string[] Documentation for this section has not yet been entered.
abstract	GetExplanation () : string Returns a textual explanation of the query. (Inherited from Query.)
	GetQueryPrefixes () : SemWeb.NamespaceManager Gets a NamespaceManager containing all of the PREFIX definitions used in the query.
abstract	Run (SemWeb.SelectableSource, QueryResultSink) Runs the query on a data source, outputting to a QueryResultSink. (Inherited from Query.)
	Run (SemWeb.SelectableSource, System.IO.TextWriter) Runs the query on a data source, outputting to a TextWriter. (Inherited from Query.)
	Select (SemWeb.SelectableSource, QueryResultSink) Executes a SELECT query on the given data source and sends the resulting variable bindings to a QueryResultSink.
	Select (SemWeb.SelectableSource, System.IO.TextWriter) Executes a SELECT query on the given data source and outputs the result of the query in the SPARQL results format to a TextWriter.