The Semantic Web is a Web of data — of dates and titles and part numbers and chemical properties and any other data one might conceive of. RDF provides the foundation for publishing and linking your data. Various technologies allow you to embed data in documents (RDFa, GRDDL) or expose what you have in SQL databases, or make it available as RDF files.
However, just as relational databases or XML need specific query languages (SQL and XQuery, respectively), the Web of Data, typically represented using RDF as a data format, needs its own, RDF-specific query language and facilities. This is provided by the SPARQLquery language and the accompanying protocols. SPARQL makes it possible to send queries and receive results, e.g., through HTTP or SOAP.
Technically, SPARQL queries are based on (triple) patterns. RDF can be seen as a set of relationships among resources (i.e., RDF triples); SPARQL queries provide one or more patterns against such relationships. These triple patterns are similar to RDF triples, except that one or more of the constituent resource references are variables. A SPARQL engine would returns the resources for all triples that match these patterns.
Using SPARQL consumers of the Web of Data can extract possibly complex information (i.e., existing resource references and their relationships) which are returned, for example, in a table format. This table can be incorporated into another Web page; using this approach SPARQL provides a powerful tool to build, for example, complex mash-up sites or search engines that include data stemming from the Semantic Web.
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