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348 lines
18 KiB
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<div class="w3c_leftCol"><h2 class="offscreen">Site Navigation</h2>
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<h3 class="category"><span class="ribbon"><a href="/standards/semanticweb/" title="Up to Semantic Web">Semantic Web <img src="/2008/site/images/header-link" alt="Header link" width="13" height="13" class="header-link" /></a></span></h3>
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<li><a href="/standards/semanticweb/applications.html">Vertical Applications</a></li>
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<li><a href="/standards/semanticweb/inference.html">Inference</a></li>
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<li><a class="current">Query</a></li>
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<li class="current">Query</li>
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<h1 class="title">
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<img src="/2008/site/images/theme-web-semantics" alt="" />Query</h1>
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<ul class="w3c_toc">
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<li class="toc_prefix">On this page → </li>
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<li><a href="#summary">what is topic</a>
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<span class="bullet">• </span>
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<li><a href="#uses">what is topic used for</a>
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<span class="bullet">• </span>
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<li><a href="#examples">examples</a>
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<li><a href="#examples">learn more</a>
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<div class="line">
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<div class="unit size3on4">
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<h2 id="summary">What is Query?</h2>
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<p>“Query” in the Semantic Web context means technologies and protocols that can programmatically
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retrieve information from the Web of Data.
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</p>
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<h2 id="uses">What is Query Used For?</h2>
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<p>
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The Semantic Web is a Web of data — of dates and titles
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and part numbers and chemical properties and any other data one
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might conceive of. RDF provides the foundation for publishing
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and linking your data. Various technologies allow you to
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embed data in documents (RDFa, GRDDL) or expose what you have
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in SQL databases, or make it available as RDF files.
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</p>
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<p>
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However, just as relational databases or XML need specific query languages (SQL and XQuery,
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respectively), the Web of Data, typically represented using RDF as a data format, needs its own,
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RDF-specific query language and facilities. This is provided by the SPARQLquery language and the accompanying
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protocols. SPARQL makes it possible to send queries and receive results, e.g., through
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HTTP or SOAP.
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</p>
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<p>Technically, SPARQL queries are based on <em>(triple) patterns</em>. RDF can be seen as a set of relationships
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among resources (i.e., RDF triples); SPARQL queries provide one or more patterns against such relationships.
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These triple patterns are similar to RDF triples, except that one or more of the constituent resource references
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are variables. A SPARQL engine would returns the resources for all triples that match these patterns.</p>
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<p>Using SPARQL consumers of the Web of Data can extract possibly complex information (i.e., existing resource references and
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their relationships) which are returned, for example, in a table format. This table can be incorporated
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into another Web page; using this approach SPARQL provides a powerful tool to build, for example, complex mash-up
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sites or search engines that include data stemming from the Semantic Web.</p>
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<h2 id="examples">Examples</h2>
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<p>The short introduction on <a href="inference#examples">inferences</a> includes a small example
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with a dataset containing the triple <tt>(Flipper isA Dolphin)</tt>. Using SPARQL the user may
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issue a query of the form <tt>(Flipper isA ?species)</tt> where <tt>?species</tt> denotes a variable. The query engine
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checks through the data and retrieves the value of <tt>Dolphin</tt> as a possible value for <tt>?species</tt> which
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constitutes a possible answer to the query. The dataset may also contain the triple <tt>(Flipper isA Mammal)</tt>
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(e.g., as a result of an <a href="inference#examples">inference</a>); in this case <tt>Mammal</tt> is also a possible
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value for <tt>?species</tt> which should also be returned as a result of the query. By providing several
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triple patterns, complex queries can be created and used by the application. </p>
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<h2 id="learnmore">Learn More</h2>
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<p>The Semantic Web community maintains a <a href="http://esw.w3.org/topic/SwBooks">list of books</a>
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on a W3C Wiki page. Some of those books
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are introductory in nature while others are conference proceedings or
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textbook that address more advanced topics.
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Details of <a href="http://www.w3.org/2007/11/Talks/Semantic_Web.html">recent and upcoming
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Semantic Web related talks</a>, given by the <a href="http://www.w3.org/People/">W3C Staff</a>,
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the <a href="http://www.w3.org/Consortium/Offices/">staff of the W3C Offices</a>,
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and members of the W3C Working Groups are available separately; the slides
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are usually publicly available. The
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W3C also maintains a <a href="http://www.w3.org/2001/sw/sweo/public/UseCases/">collection of Semantic
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Web Case Studies and Use Cases</a> that
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show how Semantic Web technologies, including queries, are used in
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practice. Finally, the <a href="/2001/sw/SW-FAQ">Semantic Web FAQ</a> may also be of help
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in understanding the various concepts.</p>
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<div id="pressreleases"></div>
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<div id="recentpress"><!-- start generated recent press --><h2>Recent Press</h2>
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<dl>
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<dt>
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<span class="press_date">29 December</span>
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<br />
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<a href="http://semanticweb.com/">semanticweb.com</a>
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</dt>
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<dd>
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<a href="http://semanticweb.com/ring-in-a-new-year-for-the-semantic-web_b25629#more-25629">
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“<cite>Ring In A New Year For the Semantic Web</cite>”
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</a>
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</dd>
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<dt>
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<span class="press_date">8 December</span>
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<br />
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<a href="http://blog.antidot.net/">Blog Antidot</a>
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</dt>
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<dd>
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<a href="http://blog.antidot.net/2011/12/08/grande-semaine-pour-lopen-data-francais/">
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“<cite>Grande semaine pour l’Open Data français !</cite>”
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</a>
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</dd>
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</dl>
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<!-- end generated recent press --></div>
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<h2 id="specifications">Current Status of Specifications</h2>
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<p>Learn more about the current status of specifications related
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to:
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</p>
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<div id="w3c_techs_body"><ul class="show_items"><li>
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<a href="/standards/techs/sparql">SPARQL</a>
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</li></ul></div>
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<p>These <a href="/Consortium/activities">W3C Groups</a> are
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working on the related specifications:
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</p>
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<div id="w3c_groups"><ul class="show_items">
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<li>
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<a href="http://www.w3.org/2001/sw/DataAccess/">SPARQL Working Group</a>
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</li>
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</ul></div>
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<div class="unit size1on4 lastUnit w3c_rhs">
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<div id="curstatusrhs">
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<h2 class="h4 category">Current Status</h2>
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<ul class="theme right-side">
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<li>
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<a href="/standards/techs/sparql">SPARQL</a>
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