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| <h1 class="title">SPARQL Tutorial - Basic Patterns</h1> |
| |
| <p>This section covers basic patterns and solutions, the main building |
| blocks of SPARQL queries.</p> |
| <h2 id="solutions">Solutions</h2> |
| <p>Query solutions are a set of pairs of a variable name with a value. |
| A <code>SELECT</code> query directly exposes the solutions (after |
| order/limit/offset are applied) as the result set - other query |
| forms use the solutions to make a graph. The solution is the way |
| the pattern matched - which values the variables must take for a |
| pattern to match.</p> |
| <p>The first query example had a single solution. Change the pattern |
| to this second query: (<a href="sparql_data/q-bp1.rq">q-bp1.rq</a>):</p> |
| <pre><code>SELECT ?x ?fname |
| WHERE {?x <http://www.w3.org/2001/vcard-rdf/3.0#FN> ?fname} |
| </code></pre> |
| <p>This has 4 solutions, one for each VCARD name property triples in |
| the data source</p> |
| <pre><code>---------------------------------------------------- |
| | x | fname | |
| ==================================================== |
| | <http://somewhere/RebeccaSmith/> | "Becky Smith" | |
| | <http://somewhere/SarahJones/> | "Sarah Jones" | |
| | <http://somewhere/JohnSmith/> | "John Smith" | |
| | <http://somewhere/MattJones/> | "Matt Jones" | |
| ---------------------------------------------------- |
| </code></pre> |
| <p>So far, with triple patterns and basic patterns, every variable |
| will be defined in every solution. The solutions to a query can be |
| thought of a table, but in the general case, it is a table where |
| not every row will have a value for every column. All the solutions |
| to a given SPARQL query don’t have to have values for all the |
| variables in every solution as we shall see later.</p> |
| <h2 id="basic-patterns">Basic Patterns</h2> |
| <p>A basic pattern is a set of triple patterns. It matches when the |
| triple patterns all match with the same value used each time the |
| variable with the same name is used.</p> |
| <pre><code>SELECT ?givenName |
| WHERE |
| { ?y <http://www.w3.org/2001/vcard-rdf/3.0#Family> "Smith" . |
| ?y <http://www.w3.org/2001/vcard-rdf/3.0#Given> ?givenName . |
| } |
| </code></pre> |
| <p>This query (<a href="sparql_data/q-bp2.rq">q-bp2.rq</a>) involves two triple patterns, each |
| triple ends in a ‘.’ (but the dot after the last one can be omitted |
| like it was in the one triple pattern example). The variable y has |
| to be the same for each triple pattern match. The solutions |
| are:</p> |
| <pre><code>------------- |
| | givenName | |
| ============= |
| | "John" | |
| | "Rebecca" | |
| ------------- |
| </code></pre> |
| <h3 id="qnames">QNames</h3> |
| <p>There is shorthand mechanism for writing long URIs using prefixes. |
| The query above is more clearly written as the query |
| (<a href="sparql_data/q-bp3.rq">q-bp3.rq</a>):</p> |
| <pre><code>PREFIX vcard: <http://www.w3.org/2001/vcard-rdf/3.0#> |
| |
| SELECT ?givenName |
| WHERE |
| { ?y vcard:Family "Smith" . |
| ?y vcard:Given ?givenName . |
| } |
| </code></pre> |
| <p>This is a prefixing mechanism - the two parts of the URIs, from the |
| prefix declaration and from the part after the “:” in the qname, |
| are concatenated together. This is strictly not what an XML qname |
| is but uses the RDF rule for turning a qname into a URI by |
| concatenating the parts.</p> |
| <h3 id="blank-nodes">Blank Nodes</h3> |
| <p>Change the query just a little to return y as well |
| (<a href="sparql_data/q-bp4.rq">q-bp4.rq</a>) :</p> |
| <pre><code>PREFIX vcard: <http://www.w3.org/2001/vcard-rdf/3.0#> |
| |
| SELECT ?y ?givenName |
| WHERE |
| { ?y vcard:Family "Smith" . |
| ?y vcard:Given ?givenName . |
| } |
| </code></pre> |
| <p>and the blank nodes appear</p> |
| <pre><code>-------------------- |
| | y | givenName | |
| ==================== |
| | _:b0 | "John" | |
| | _:b1 | "Rebecca" | |
| -------------------- |
| </code></pre> |
| <p>as odd looking qnames starting _:. This isn’t the internal label |
| for the blank node - it is ARQ printing them out that assigned the |
| _:b0, _:b1 to show when two blank nodes are the same. Here they |
| are different. It does not reveal the internal label used for the |
| blank node although that is available when using the Java API.</p> |
| <p><a href="sparql_filters.html">Next: Filters</a></p> |
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