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| <h1 class="title">ARQ - Generate JSON from SPARQL</h1> |
| |
| |
| <main class="d-flex flex-xl-row flex-column"> |
| |
| <aside class="text-muted align-self-start mb-3 p-0 d-xl-none d-block"> |
| <h2 class="h6 sticky-top m-0 p-2 bg-body-tertiary">On this page</h2> |
| <nav id="TableOfContents"> |
| <ul> |
| <li><a href="#query-syntax">Query Syntax</a> |
| <ul> |
| <li><a href="#grammar">Grammar</a></li> |
| </ul> |
| </li> |
| <li><a href="#programming-api">Programming API</a></li> |
| <li><a href="#fuseki-support">Fuseki Support</a></li> |
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| <article class="flex-column me-lg-4"> |
| <p>The current W3C recommendation of |
| <a href="http://www.w3.org/TR/sparql11-query/">SPARQL 1.1</a> supports the <a href="https://www.w3.org/TR/2013/REC-sparql11-results-json-20130321/">query results in |
| JSON format</a>. What is |
| described in this page is not that format, but an extension of Apache Jena, which allows |
| users to define how results should be returned in a key/value pair fashion, providing |
| this way a simpler output. This output can be easily used as model for web applications, |
| or inspecting data.</p> |
| <p>Compare the output of this extension:</p> |
| <pre><code>[ |
| { |
| "book": "http://example.org/book/book6", |
| "title": "Harry Potter and the Half-Blood Prince" |
| }, |
| { |
| "book": "http://example.org/book/book7", |
| "title": "Harry Potter and the Deathly Hallows" |
| }, |
| ] |
| </code></pre> |
| <p>With the output of the SPARQL 1.1 query result JSON format below:</p> |
| <pre><code>{ |
| "head": { "vars": [ "book" , "title" ] |
| } , |
| "results": { |
| "bindings": [ |
| { |
| "book": { "type": "uri" , "value": "http://example.org/book/book6" } , |
| "title": { "type": "literal" , "value": "Harry Potter and the Half-Blood Prince" } |
| } , |
| { |
| "book": { "type": "uri" , "value": "http://example.org/book/book7" } , |
| "title": { "type": "literal" , "value": "Harry Potter and the Deathly Hallows" } |
| } |
| ] |
| } |
| } |
| </code></pre> |
| <p>This feature was added in Jena 3.8.0.</p> |
| <h2 id="query-syntax">Query Syntax</h2> |
| <p>The <code>JSON</code> syntax is similar in certain ways to the SPARQL <code>CONSTRUCT</code> syntax.</p> |
| <pre><code>PREFIX purl: <http://purl.org/dc/elements/1.1/> |
| PREFIX w3: <http://www.w3.org/2001/vcard-rdf/3.0#> |
| PREFIX : <http://example.org/book/> |
| |
| JSON { |
| "author": ?author, |
| "title": ?title |
| } |
| WHERE |
| { |
| ?book purl:creator ?author . |
| ?book purl:title ?title . |
| FILTER (?author = 'J.K. Rowling') |
| } |
| </code></pre> |
| <p>As in <code>CONSTRUCT</code>, users are able to specify how the output must look like, using a simple |
| key/value pair pattern, which could produce the following output for the query above.</p> |
| <pre><code>[ |
| { |
| "author" : "J.K. Rowling" , |
| "title" : "Harry Potter and the Deathly Hallows" |
| } |
| { |
| "author" : "J.K. Rowling" , |
| "title" : "Harry Potter and the Philosopher's Stone" |
| } |
| { |
| "author" : "J.K. Rowling" , |
| "title" : "Harry Potter and the Order of the Phoenix" |
| } |
| { |
| "author" : "J.K. Rowling" , |
| "title" : "Harry Potter and the Half-Blood Prince" |
| } |
| ] |
| </code></pre> |
| <h3 id="grammar">Grammar</h3> |
| <p>The normative definition of the syntax grammar of the query string is defined in this table:</p> |
| <table> |
| <thead> |
| <tr> |
| <th>Rule</th> |
| <th></th> |
| <th>Expression</th> |
| </tr> |
| </thead> |
| <tbody> |
| <tr> |
| <td>JsonQuery</td> |
| <td>::=</td> |
| <td>JsonClause ( DatasetClause )* WhereClause SolutionModifier</td> |
| </tr> |
| <tr> |
| <td>JsonClause</td> |
| <td>::=</td> |
| <td>‘JSON’ ‘{’ JsonObjectMember ( ‘,’ JsonObjectMember )* ‘}’</td> |
| </tr> |
| <tr> |
| <td>JsonObjectMember</td> |
| <td>::=</td> |
| <td>String ‘:’ ( Var | RDFLiteral | NumericLiteral | BooleanLiteral )</td> |
| </tr> |
| </tbody> |
| </table> |
| <p><code>DatasetClause</code>, <code>WhereClause</code>, <code>SolutionModifier</code>, <code>String</code>, <code>Var</code>, ‘RDFLiteral’, |
| <code>NumericLiteral</code>, and ‘BooleanLiteral’ are as for the <a href="http://www.w3.org/TR/sparql11-query/#grammar">SPARQL 1.1 Grammar</a></p> |
| <h2 id="programming-api">Programming API</h2> |
| <p>ARQ provides 2 additional methods in <a href="/documentation/javadoc/arq/org.apache.jena.arq/org/apache/jena/query/QueryExecution.html">QueryExecution</a> for JSON.</p> |
| <pre><code>Iterator<JsonObject> QueryExecution.execJsonItems() |
| JsonArray QueryExecution.execJson() |
| </code></pre> |
| <p>In order to use these methods, it’s required to switch on the query syntax |
| of ARQ beforehand, when creating the <code>Query</code> object:</p> |
| <pre><code>Query query = QueryFactory.create(queryString, Syntax.syntaxARQ) |
| String queryString = "JSON { 'name' : ?name, 'age' : ?age } WHERE ... " |
| ... |
| Iterator<JsonObject> json = qexec.execJsonItems() |
| </code></pre> |
| <h2 id="fuseki-support">Fuseki Support</h2> |
| <p>Users are able to use Fuseki web interface, as well as the other HTTP endpoints to submit |
| queries using any programming language. The following example shows how to POST to the query |
| endpoint passing the query as a form data field.</p> |
| <pre><code>curl -XPOST --data "query=JSON { 'name' : ?name, 'age': ?age } WHERE { ... }" http://localhost:3030/ds/query |
| </code></pre> |
| <p>The web interface editor parses the SPARQL implementation syntax, so syntax errors are expected |
| in the web editor at this moment when using the <code>JSON</code> clause. The query should still be correctly |
| executed, and the results displayed as with other normal SPARQL queries.</p> |
| <p><a href="index.html">ARQ documentation index</a></p> |
| |
| </article> |
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| <nav id="TableOfContents"> |
| <ul> |
| <li><a href="#query-syntax">Query Syntax</a> |
| <ul> |
| <li><a href="#grammar">Grammar</a></li> |
| </ul> |
| </li> |
| <li><a href="#programming-api">Programming API</a></li> |
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