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| <h1 class="title">ARQ Property Paths</h1> |
| |
| <h1 id="arq---property-paths">ARQ - Property Paths</h1> |
| <p>A property path is a possible route through a graph between two |
| graph nodes. A trivial case is a property path of length exactly |
| one, which is a triple pattern.</p> |
| <p>Most property paths are now legal SPARQL 1.1 syntax, there are some advanced property |
| paths which are syntactic extensions and are only available if the query is parsed with language <code>Syntax.syntaxARQ</code>.</p> |
| <h2 id="path-language">Path Language</h2> |
| <p>A property path expression (or just ‘path’) is similar to a string |
| regular expression but over properties, not characters. ARQ |
| determines all matches of a path expression and binds subject or |
| object as appropriate. Only one match is recorded - no duplicates |
| for any given path expression, although if the path is used in a |
| situation where it’s initial points is already repeated in a |
| pattern, then this duplication is preserved.</p> |
| <table> |
| <thead> |
| <tr> |
| <th>Path example</th> |
| <th>Meaning</th> |
| </tr> |
| </thead> |
| <tbody> |
| <tr> |
| <td><tt>dc:title | rdfs:label</tt></td> |
| <td>Dublin Core title or an RDFS label.</td> |
| </tr> |
| <tr> |
| <td><code>foaf:knows/foaf:name</code></td> |
| <td>Name of people one “knows” steps away.</td> |
| </tr> |
| <tr> |
| <td><code>foaf:knows/foaf:knows/foaf:name</code></td> |
| <td>Name of people two “knows” steps away.</td> |
| </tr> |
| </tbody> |
| </table> |
| <p>In the description below, <em><code>uri</code></em> is either a URI or a prefixed |
| name.</p> |
| <table> |
| <thead> |
| <tr> |
| <th>Syntax Form</th> |
| <th>Matches</th> |
| </tr> |
| </thead> |
| <tbody> |
| <tr> |
| <td><tt><i>uri</i></tt></td> |
| <td>A URI or a prefixed name. A path of length one.</td> |
| </tr> |
| <tr> |
| <td><tt>^<i>elt</i></tt></td> |
| <td>Reverse path (object to subject)</td> |
| </tr> |
| <tr> |
| <td><tt>(<i>elt</i>)</tt></td> |
| <td>A group path <em><code>elt</code></em>, brackets control precedence.</td> |
| </tr> |
| <tr> |
| <td><tt><i>elt1</i> / <i>elt2</i></tt></td> |
| <td>A sequence path of <em><code>elt1</code></em>, followed by <em><code>elt2</code></em></td> |
| </tr> |
| <tr> |
| <td><tt><i>elt1</i> | <i>elt2</i></tt></td> |
| <td>A alternative path of <tt><i>elt1</i></tt>, or <tt><i>elt2</i></tt> (both possibilities are tried)</td> |
| </tr> |
| <tr> |
| <td><tt><i>elt</i>*</tt></td> |
| <td>A path of zero or more occurrences of <em><code>elt</code></em>.</td> |
| </tr> |
| <tr> |
| <td><tt><i>elt</i>+</tt></td> |
| <td>A path of one or more occurrences of <em><code>elt</code></em>.</td> |
| </tr> |
| <tr> |
| <td><tt><i>elt</i>?</tt></td> |
| <td>A path of zero or one <em><code>elt</code></em>.</td> |
| </tr> |
| <tr> |
| <td><tt>!<i>uri</i></tt></td> |
| <td>A path matching a property which isn’t <tt><i>uri</i></tt> (negated property set)</td> |
| </tr> |
| <tr> |
| <td><tt>!(<i>uri1</i>|…|<i>uriN</i>)</tt></td> |
| <td>A path matching a property which isn’t any of <code>uri1 ... uri</code> (negated property set)</td> |
| </tr> |
| </tbody> |
| </table> |
| <p>ARQ extensions: to use these you must use <code>Syntax.syntaxARQ</code></p> |
| <table> |
| <thead> |
| <tr> |
| <th>Syntax Form</th> |
| <th>Matches</th> |
| </tr> |
| </thead> |
| <tbody> |
| <tr> |
| <td><tt><i>elt1</i> ^ <i>elt2</i></tt></td> |
| <td>Shorthand for <tt><i>elt1 / ^elt2</i></tt>, that is <em><code>elt1</code></em> followed by reverse <em><code>elt2</code></em>.</td> |
| </tr> |
| <tr> |
| <td><i><tt>elt</tt></i><code>{n,m}</code></td> |
| <td>A path between n and m occurrences of <em><code>elt</code></em>.</td> |
| </tr> |
| <tr> |
| <td><i><tt>elt</tt></i><code>{n}</code></td> |
| <td>Exactly <code>n</code> occurrences of <code>elt</code>. A fixed length path.</td> |
| </tr> |
| <tr> |
| <td><i><tt>elt</tt></i><code>{n,}</code></td> |
| <td><code>n</code> or more occurrences of <em><code>elt</code></em>.</td> |
| </tr> |
| <tr> |
| <td><i><tt>elt</tt></i><code>{,n}</code></td> |
| <td>Between 0 and <code>n</code> occurrences of <em><code>elt</code></em>.</td> |
| </tr> |
| </tbody> |
| </table> |
| <p>Precedence:</p> |
| <ol> |
| <li>URI, prefixed names</li> |
| <li>Negated property set</li> |
| <li>Groups</li> |
| <li>Unary ^ reverse links</li> |
| <li>Unary operators <code>*</code>, <code>?</code>, <code>+</code> and <code>{}</code> forms</li> |
| <li>Binary operators <code>/</code> and ^</li> |
| <li>Binary operator <code>|</code></li> |
| </ol> |
| <p>Precedence is left-to-right within groups.</p> |
| <h2 id="path-evaluation">Path Evaluation</h2> |
| <p>Paths are “simple” if they involve only operators / (sequence), ^ |
| (reverse, unary or binary) and the form {<em>n</em>}, for some single |
| integer <em>n</em>. Such paths are fixed length. They are translated to |
| triple patterns by the query compiler and do not require special |
| path-evaluation at runtime.</p> |
| <p>A path of just a URI is still a single triple pattern.</p> |
| <p>A path is “complex” if it involves one or more of the operators |
| *,?, + and {}. Such paths require special evaluation and provide |
| expressivity outside of strict SPARQL because paths can be of |
| variable length. When used with models backed by SQL databases, |
| complex path expressions may take some time.</p> |
| <p>A path of length zero connects a graph node to itself.</p> |
| <p>Cycles in paths are possible and are handled.</p> |
| <p>Paths do not need to be anchored at one end of the other, although |
| this can lead to large numbers of result because the whole graph is |
| searched.</p> |
| <p>Property functions in paths are only available for simple paths.</p> |
| <h2 id="extended-language">Extended Language</h2> |
| <p>This involves is syntactic extension and is available if the query |
| is parsed with language <code>Syntax.syntaxARQ</code>.</p> |
| <p>Paths can be directly included in the query in the property |
| position of a triple pattern:</p> |
| <pre><code>PREFIX : <http://example/> |
| PREFIX rdf: <http://www.w3.org/1999/02/22-rdf-syntax-ns#> |
| PREFIX rdfs: <http://www.w3.org/2000/01/rdf-schema#> |
| |
| # Find the types of :x, following subClassOf |
| SELECT * |
| { |
| :x rdf:type/rdfs:subClassOf* ?t |
| } |
| </code></pre> |
| <h2 id="examples">Examples</h2> |
| <h3 id="simple-paths">Simple Paths</h3> |
| <p>Find the name of any people that Alice knows.</p> |
| <pre><code>{ |
| ?x foaf:mbox <mailto:alice@example> . |
| ?x foaf:knows/foaf:name ?name . |
| } |
| </code></pre> |
| <p>Find the names of people 2 “<code>foaf:knows</code>” links away.</p> |
| <pre><code>{ |
| ?x foaf:mbox <mailto:alice@example> . |
| ?x foaf:knows/foaf:knows/foaf:name ?name . |
| } |
| </code></pre> |
| <p>This is the same as the strict SPARQL query:</p> |
| <pre><code>{ |
| ?x foaf:mbox <mailto:alice@example> . |
| ?x foaf:knows [ foaf:knows [ foaf:name ?name ]]. |
| } |
| </code></pre> |
| <p>or, with explicit variables:</p> |
| <pre><code>{ |
| ?x foaf:mbox <mailto:alice@example> . |
| ?x foaf:knows ?a1 . |
| ?a1 foaf:knows ?a2 . |
| ?a2 foaf:name ?name . |
| } |
| </code></pre> |
| <p>Because someone Alice knows may well know Alice, the example above |
| may include Alice herself. This could be avoided with:</p> |
| <pre><code>{ ?x foaf:mbox <mailto:alice@example> . |
| ?x foaf:knows/foaf:knows ?y . |
| FILTER ( ?x != ?y ) |
| ?y foaf:name ?name |
| } |
| </code></pre> |
| <p>These two are the same query: the second is just reversing the |
| property direction which swaps the roles of subject and object.</p> |
| <pre><code>{ ?x foaf:mbox <mailto:alice@example> } |
| |
| { <mailto:alice@example> ^foaf:mbox ?x } |
| </code></pre> |
| <p>Mutual <code>foaf:knows</code> relationships: <code>?x</code> knows someone who knows |
| <code>?x</code></p> |
| <pre><code>{ |
| ?x foaf:knows^foaf:knows ?x . |
| } |
| </code></pre> |
| <p>Negated property sets define matching by naming one or more |
| properties that must not match. Match if there is a triple from |
| <code>?x</code> to <code>?y</code> which is not <code>rdf:type</code>.</p> |
| <pre><code>{ |
| ?x !rdf:type ?y . |
| } |
| |
| { |
| ?x !(rdf:type|^rdf:type) ?y . |
| } |
| </code></pre> |
| <p>Only properties and reverse properties are allowed in a negated |
| property set, not a full path expression.</p> |
| <h3 id="complex-paths">Complex Paths</h3> |
| <p>Find the names of all the people can be reached from Alice by |
| <code>foaf:knows</code>:</p> |
| <pre><code>{ |
| ?x foaf:mbox <mailto:alice@example> . |
| ?x foaf:knows+/foaf:name ?name . |
| } |
| </code></pre> |
| <p>Again, because of cycles in <code>foaf:knows</code> relationships, it is |
| likely to include Alice herself.</p> |
| <p>Some forms of limited inference are possible as well. For example: |
| all types and supertypes of a resource:</p> |
| <pre><code>{ <http://example/> rdf:type/rdfs:subClassOf* ?type } |
| </code></pre> |
| <p>All resources and all their inferred types:</p> |
| <pre><code>{ ?x rdf:type/rdfs:subClassOf* ?type } |
| </code></pre> |
| <h2 id="use-with-legal-sparql-syntax">Use with Legal SPARQL Syntax</h2> |
| <p>A path can parsed, then installed as a |
| <a href="extension.html#propertyFunctions">property function</a> to be |
| referred to by URI. This way, when the URI is used in the predicate |
| location in a triple pattern, the path expression is evaluated.</p> |
| <pre><code>Path path = ... |
| String uri = ... |
| PathLib.install(uri, path) ; |
| </code></pre> |
| <p>For example:</p> |
| <pre><code>Path path = PathParser.parse("rdf:type/rdfs:subClassOf*", PrefixMapping.Standard) ; |
| String uri = "http://example/ns#myType" ; |
| PathLib.install(uri, path) ; |
| </code></pre> |
| <p>and the SPARQL query:</p> |
| <pre><code>PREFIX : <http://example/> |
| PREFIX ns: <http://example/ns#> |
| # Find the types of :x, following subClassOf |
| SELECT * { :x ns:myType ?t} |
| </code></pre> |
| <p>This also works with if an existing property is redefined (a URI in |
| a path expression is not interpreted as a property function) so, |
| for example, <code>rdf:type</code> can be redefined as a path that also |
| considers RDFS sub -class relationships. The path is a complex path |
| so the property function for <code>rdf:type</code> is not triggered.</p> |
| <pre><code>Path path = PathParser.parse("rdf:type/rdfs:subClassOf*", PrefixMapping.Standard) ; |
| PathLib.install(RDF.type.getURI(), path) ; |
| </code></pre> |
| <p><a href="index.html">ARQ documentation index</a></p> |
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