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| <h1 class="title">SPARQL Tutorial - Optional Information</h1> |
| |
| <p>RDF is semi-structured data so SPARQL has a the ability to query |
| for data but not to fail query when that data does not exist. The |
| query is using an optional part to extend the information found in |
| a query solution but to return the non-optional information |
| anyway.</p> |
| <h2 id="optionals">OPTIONALs</h2> |
| <p>This query (<a href="sparql_data/q-opt1.rq">q-opt1.rq</a>) gets the name of a person and |
| also their age if that piece of information is available.</p> |
| <pre><code>PREFIX info: <http://somewhere/peopleInfo#> |
| PREFIX vcard: <http://www.w3.org/2001/vcard-rdf/3.0#> |
| |
| SELECT ?name ?age |
| WHERE |
| { |
| ?person vcard:FN ?name . |
| OPTIONAL { ?person info:age ?age } |
| } |
| </code></pre> |
| <p>Two of the four people in the data (<a href="sparql_data/vc-db-2.rdf">vc-db-2.rdf</a>)have |
| age properties so two of the query solutions have that |
| information. However, because the triple pattern for the age is |
| optional, there is a pattern solution for the people who don’t have |
| age information.</p> |
| <pre><code>------------------------ |
| | name | age | |
| ======================= |
| | "Becky Smith" | 23 | |
| | "Sarah Jones" | | |
| | "John Smith" | 25 | |
| | "Matt Jones" | | |
| ----------------------- |
| </code></pre> |
| <p>If the optional clause had not been there, no age information would |
| have been retrieved. If the triple pattern had been included but |
| not optional then we would have the query |
| (<a href="sparql_data/q-opt2.rq">q-opt2.rq</a>):</p> |
| <pre><code>PREFIX info: <http://somewhere/peopleInfo#> |
| PREFIX vcard: <http://www.w3.org/2001/vcard-rdf/3.0#> |
| |
| SELECT ?name ?age |
| WHERE |
| { |
| ?person vcard:FN ?name . |
| ?person info:age ?age . |
| } |
| </code></pre> |
| <p>with only two solutions:</p> |
| <pre><code>----------------------- |
| | name | age | |
| ======================= |
| | "Becky Smith" | 23 | |
| | "John Smith" | 25 | |
| ----------------------- |
| </code></pre> |
| <p>because the <code>info:age</code> property must now be present in a solution.</p> |
| <h2 id="optionals-with-filters">OPTIONALs with FILTERs</h2> |
| <p><code>OPTIONAL</code> is a binary operator that combines two graph patterns. |
| The optional pattern is any group pattern and may involve any |
| SPARQL pattern types. If the group matches, the solution is |
| extended, if not, the original solution is given |
| (<a href="sparql_data/q-opt3.rq">q-opt3.rq</a>).</p> |
| <pre><code>PREFIX info: <http://somewhere/peopleInfo#> |
| PREFIX vcard: <http://www.w3.org/2001/vcard-rdf/3.0#> |
| |
| SELECT ?name ?age |
| WHERE |
| { |
| ?person vcard:FN ?name . |
| OPTIONAL { ?person info:age ?age . FILTER ( ?age > 24 ) } |
| } |
| </code></pre> |
| <p>So, if we filter for ages greater than 24 in the optional part, we |
| will still get 4 solutions (from the <code>vcard:FN</code> pattern) but only |
| get ages if they pass the test.</p> |
| <pre><code>----------------------- |
| | name | age | |
| ======================= |
| | "Becky Smith" | | |
| | "Sarah Jones" | | |
| | "John Smith" | 25 | |
| | "Matt Jones" | | |
| ----------------------- |
| </code></pre> |
| <p>No age included for “Becky Smith” because it is less than 24.</p> |
| <p>If the filter condition is moved out of the optional part, then it |
| can influence the number of solutions but it may be necessary to |
| make the filter more complicated to allow for variable <code>age</code> being |
| unbound (<a href="sparql_data/q-opt4.rq">q-opt4.rq</a>).</p> |
| <pre><code>PREFIX info: <http://somewhere/peopleInfo#> |
| PREFIX vcard: <http://www.w3.org/2001/vcard-rdf/3.0#> |
| |
| SELECT ?name ?age |
| WHERE |
| { |
| ?person vcard:FN ?name . |
| OPTIONAL { ?person info:age ?age . } |
| FILTER ( !bound(?age) || ?age > 24 ) |
| } |
| </code></pre> |
| <p>If a solution has an <code>age</code> variable, then it must be greater than |
| 24. It can also be unbound. There are now three solutions:</p> |
| <pre><code>----------------------- |
| | name | age | |
| ======================= |
| | "Sarah Jones" | | |
| | "John Smith" | 25 | |
| | "Matt Jones" | | |
| ----------------------- |
| </code></pre> |
| <p>Evaluating an expression which has an unbound variables where a |
| bound one was expected causes an evaluation exception and the whole |
| expression fails.</p> |
| <h2 id="optionals-and-order-dependent-queries">OPTIONALs and Order Dependent Queries</h2> |
| <p>One thing to be careful of is using the same variable in two or |
| more optional clauses (and not in some basic pattern as well):</p> |
| <pre><code>PREFIX foaf: <http://xmlns.com/foaf/0.1/> |
| PREFIX vCard: <http://www.w3.org/2001/vcard-rdf/3.0#> |
| |
| SELECT ?name |
| WHERE |
| { |
| ?x a foaf:Person . |
| OPTIONAL { ?x foaf:name ?name } |
| OPTIONAL { ?x vCard:FN ?name } |
| } |
| </code></pre> |
| <p>If the first optional binds <code>?name</code> and <code>?x</code> to some values, the |
| second <code>OPTIONAL</code> is an attempt to match the ground triples (<code>?x</code> |
| and <code>?name</code> have values). If the first optional did not |
| match the optional part, then the second one is an attempt to match |
| its triple with two variables.</p> |
| <p>With an example set of data in which every combination of values exist:</p> |
| <div class="highlight"><pre style="background-color:#f8f8f8;-moz-tab-size:4;-o-tab-size:4;tab-size:4"><code class="language-xml" data-lang="xml"><span style="color:#008000;font-weight:bold"><rdf:RDF</span> |
| <span style="color:#b44">xmlns:rdf=</span><span style="color:#b44">'http://www.w3.org/1999/02/22-rdf-syntax-ns#'</span> |
| <span style="color:#b44">xmlns:vCard=</span><span style="color:#b44">'http://www.w3.org/2001/vcard-rdf/3.0#'</span> |
| <span style="color:#b44">xmlns:info=</span><span style="color:#b44">'http://somewhere/peopleInfo#'</span> |
| <span style="color:#b44">xmlns:foaf=</span><span style="color:#b44">'http://xmlns.com/foaf/0.1/'</span> |
| <span style="color:#008000;font-weight:bold">></span> |
| |
| <span style="color:#080;font-style:italic"><!-- both vCard:FN and foaf:name have values, and the values are the same --></span> |
| <span style="color:#008000;font-weight:bold"><foaf:Person</span> <span style="color:#b44">rdf:about=</span><span style="color:#b44">"http://somewhere/JohnSmith"</span><span style="color:#008000;font-weight:bold">></span> |
| <span style="color:#008000;font-weight:bold"><vCard:FN></span>John Smith<span style="color:#008000;font-weight:bold"></vCard:FN></span> |
| <span style="color:#008000;font-weight:bold"><foaf:name></span>John Smith<span style="color:#008000;font-weight:bold"></foaf:name></span> |
| <span style="color:#008000;font-weight:bold"></foaf:Person></span> |
| |
| <span style="color:#080;font-style:italic"><!-- both vCard:FN and foaf:name have values, but the values are not the same --></span> |
| <span style="color:#008000;font-weight:bold"><foaf:Person</span> <span style="color:#b44">rdf:about=</span><span style="color:#b44">"http://somewhere/RebeccaSmith"</span><span style="color:#008000;font-weight:bold">></span> |
| <span style="color:#008000;font-weight:bold"><vCard:FN></span>Becky Smith<span style="color:#008000;font-weight:bold"></vCard:FN></span> |
| <span style="color:#008000;font-weight:bold"><foaf:name></span>Rebecca Smith<span style="color:#008000;font-weight:bold"></foaf:name></span> |
| <span style="color:#008000;font-weight:bold"></foaf:Person></span> |
| |
| <span style="color:#080;font-style:italic"><!-- only vCard:FN has values --></span> |
| <span style="color:#008000;font-weight:bold"><foaf:Person</span> <span style="color:#b44">rdf:about=</span><span style="color:#b44">"http://somewhere/SarahJones"</span><span style="color:#008000;font-weight:bold">></span> |
| <span style="color:#008000;font-weight:bold"><vCard:FN></span>Sarah Jones<span style="color:#008000;font-weight:bold"></vCard:FN></span> |
| <span style="color:#008000;font-weight:bold"></foaf:Person></span> |
| |
| <span style="color:#080;font-style:italic"><!-- only foaf:name has values --></span> |
| <span style="color:#008000;font-weight:bold"><foaf:Person</span> <span style="color:#b44">rdf:about=</span><span style="color:#b44">"http://somewhere/MattJones"</span><span style="color:#008000;font-weight:bold">></span> |
| <span style="color:#008000;font-weight:bold"><foaf:name></span>Matthew Jones<span style="color:#008000;font-weight:bold"></foaf:name></span> |
| <span style="color:#008000;font-weight:bold"></foaf:Person></span> |
| |
| <span style="color:#080;font-style:italic"><!-- neither vCard:FN nor foaf:name have values --></span> |
| <span style="color:#008000;font-weight:bold"><foaf:Person</span> <span style="color:#b44">rdf:about=</span><span style="color:#b44">"http://somewhere/AdamJones"</span> <span style="color:#008000;font-weight:bold">/></span> |
| |
| <span style="color:#008000;font-weight:bold"></rdf:RDF></span> |
| </code></pre></div><p>Executing the above query will yield these solutions:</p> |
| <pre><code>------------------- |
| | name | |
| =================== |
| | "John Smith" | |
| | "Matthew Jones" | |
| | "Sarah Jones" | |
| | | |
| | "Rebecca Smith" | |
| ------------------- |
| </code></pre> |
| <p><a href="sparql_union.html">Next: union queries</a></p> |
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