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| <h1 class="title">SPARQL Tutorial - Filters</h1> |
| |
| <p>Graph matching allows patterns in the graph to be found. This |
| section describes how the values in a solution can be restricted. |
| There are many comparisons available - we just cover two cases |
| here.</p> |
| <h2 id="string-matching">String Matching</h2> |
| <p>SPARQL provides an operation to test strings, based on regular |
| expressions. This includes the ability to ask SQL “LIKE” style |
| tests, although the syntax of the regular expression is different |
| from SQL.</p> |
| <p>The syntax is:</p> |
| <pre><code>FILTER regex(?x, "pattern" [, "flags"]) |
| </code></pre> |
| <p>The flags argument is optional. The flag “i” means a |
| case-insensitive pattern match is done.</p> |
| <p>The example query (<a href="sparql_data/q-f1.rq">q-f1.rq</a>) finds given names with an |
| “r” or “R” in them.</p> |
| <pre><code>PREFIX vcard: <http://www.w3.org/2001/vcard-rdf/3.0#> |
| |
| SELECT ?g |
| WHERE |
| { ?y vcard:Given ?g . |
| FILTER regex(?g, "r", "i") } |
| </code></pre> |
| <p>with the results:</p> |
| <pre><code>------------- |
| | g | |
| ============= |
| | "Rebecca" | |
| | "Sarah" | |
| ------------- |
| </code></pre> |
| <p>The regular expression language is the same as the |
| <a href="http://www.w3.org/TR/xpath-functions/#regex-syntax">XQuery regular expression language</a> |
| which is codified version of that found in Perl.</p> |
| <h2 id="testing-values">Testing Values</h2> |
| <p>There are times when the application wants to filter on the value |
| of a variable. In the data file <a href="sparql_data/vc-db-2.rdf">vc-db-2.rdf</a>, we |
| have added an extra field for age. Age is not defined by the vCard |
| schema so we have created a new property for the purpose of this |
| tutorial. RDF allows such mixing of different definitions of |
| information because URIs are unique. Note also that the <code>info:age</code> |
| property value is typed.</p> |
| <p>In this extract of the data, we show the typed value. It can also |
| be written plain 23.</p> |
| <pre><code><http://somewhere/RebeccaSmith/> |
| info:age "23"^^xsd:integer ; |
| vCard:FN "Becky Smith" ; |
| vCard:N [ vCard:Family "Smith" ; |
| vCard:Given "Rebecca" ] . |
| </code></pre> |
| <p>So, a query (<a href="sparql_data/q-f2.rq">q-f2.rq</a>) to find the names of people who |
| are older than 24 is:</p> |
| <pre><code>PREFIX info: <http://somewhere/peopleInfo#> |
| |
| SELECT ?resource |
| WHERE |
| { |
| ?resource info:age ?age . |
| FILTER (?age >= 24) |
| } |
| </code></pre> |
| <p>The arithmetic expression must be in parentheses (round brackets). |
| The only solution is:</p> |
| <pre><code>--------------------------------- |
| | resource | |
| ================================= |
| | <http://somewhere/JohnSmith/> | |
| --------------------------------- |
| </code></pre> |
| <p>Just one match, resulting in the resource URI for John Smith. |
| Turning this round to ask for those less than 24 also yields one |
| match for Rebecca Smith. Nothing about the Jones’s.</p> |
| <p>The database contains no age information about the Jones: there are |
| no info:age properties on these vCards so the variable <code>age</code> did |
| not get a value and so was not tested by the filter.</p> |
| <p><a href="sparql_optionals.html">Next: Optionals</a></p> |
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