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| <h1 class="title">Producing Result Sets</h1> |
| |
| <p>SPARQL has four result forms:</p> |
| <ul> |
| <li>SELECT – Return a table of results.</li> |
| <li>CONSTRUCT – Return an RDF graph, based on a template in the |
| query.</li> |
| <li>DESCRIBE – Return an RDF graph, based on what the query |
| processor is configured to return.</li> |
| <li>ASK – Ask a boolean query.</li> |
| </ul> |
| <p>The SELECT form directly returns a table of solutions as a result |
| set, while DESCRIBE and CONSTRUCT use the outcome of matching to |
| build RDF graphs.</p> |
| <h2 id="solution-modifiers">Solution Modifiers</h2> |
| <p>Pattern matching produces a set of solutions. This set can be |
| modified in various ways:</p> |
| <ul> |
| <li>Projection - keep only selected variables</li> |
| <li>OFFSET/LIMIT - chop the number solutions (best used with ORDER |
| BY)</li> |
| <li>ORDER BY - sorted results</li> |
| <li>DISTINCT - yield only one row for one combination of variables |
| and values.</li> |
| </ul> |
| <p>The solution modifiers OFFSET/LIMIT and ORDER BY always apply to |
| all result forms. </p> |
| <h3 id="offset-and-limit">OFFSET and LIMIT</h3> |
| <p>A set of solutions can be abbreviated by specifying the offset (the |
| start index) and the limit (the number of solutions) to be |
| returned. Using LIMIT alone can be useful to ensure not too many |
| solutions are returned, to restrict the effect of some unexpected |
| situation. LIMIT and OFFSET can be used in conjunction with |
| sorting to take a defined slice through the solutions found.</p> |
| <h3 id="order-by">ORDER BY</h3> |
| <p>SPARQL solutions are sorted by expression, including custom |
| functions.</p> |
| <pre><code>ORDER BY ?x ?y |
| |
| ORDER BY DESC(?x) |
| |
| ORDER BY x:func(?x) # Custom sorting condition |
| </code></pre> |
| <h3 id="distinct">DISTINCT</h3> |
| <p>The SELECT result form can take the DISTINCT modifier which ensures |
| that no two solutions returned are the same - this takes place |
| after projection to the requested variables.</p> |
| <p> </p> |
| <h2 id="select">SELECT</h2> |
| <p>The <code>SELECT</code> result form is a projection, with DISTINCT applied, of |
| the solution set. <code>SELECT</code> identifies which named variables are in |
| the result set. This may be “<code>*</code>” meaning “all named variables” |
| (blank nodes in the query act like variables for matching but are |
| never returned).</p> |
| <h2 id="construct">CONSTRUCT</h2> |
| <p>CONSTRUCT builds an RDF based on a graph template. The graph |
| template can have variables which are bound by a WHERE clause. The |
| effect is to calculate the graph fragment, given the template, for |
| each solution from the WHERE clause, after taking into account any |
| solution modifiers. The graph fragments, one per solution, are |
| merged into a single RDF graph which is the result.</p> |
| <p>Any blank nodes explicitly mentioned in the graph template are |
| created afresh for each time the template is used for a solution.</p> |
| <h2 id="describe">DESCRIBE</h2> |
| <p>The CONSTRUCT form, takes an application template for the graph |
| results. The DESCRIBE form also creates a graph but the form of |
| that graph is provided the query processor, not the application. |
| For each URI found, or explicitly mentioned in the DESCRIBE clause, |
| the query processor should provide a useful fragment of RDF, such |
| as all the known details of a book. ARQ allows domain-specific |
| description handlers to be written.</p> |
| <h2 id="ask">ASK</h2> |
| <p>The ASK result form returns a boolean, true of the pattern matched |
| otherwise false.</p> |
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