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| <h1 class="title">ARQ - Aggregates</h1> |
| |
| |
| <main class="d-flex flex-xl-row flex-column"> |
| |
| <aside class="text-muted align-self-start mb-3 p-0 d-xl-none d-block"> |
| <h2 class="h6 sticky-top m-0 p-2 bg-body-tertiary">On this page</h2> |
| <nav id="TableOfContents"> |
| <ul> |
| <li><a href="#group-by">GROUP BY</a></li> |
| <li><a href="#having">HAVING</a></li> |
| <li><a href="#aggregation">Aggregation</a></li> |
| </ul> |
| </nav> |
| </aside> |
| <article class="flex-column me-lg-4"> |
| <p>ARQ includes support for GROUP BY and counting. This was previously an ARQ extension but is now legal SPARQL 1.1</p> |
| <h2 id="group-by">GROUP BY</h2> |
| <p>A <code>GROUP BY</code> clause transforms a result set so that only one row |
| will appear for each unique set of grouping variables. All other |
| variables from the query pattern are projected away and are not |
| available in the <code>SELECT</code> clause.</p> |
| <pre><code>PREFIX |
| |
| SELECT ?p ?q |
| { . . . |
| } |
| GROUP BY ?p ?q |
| </code></pre> |
| <p><code>SELECT *</code> will include variables from the <code>GROUP BY</code> but no |
| others. This ensures that results are always the same - including |
| other variables from the pattern would involve choosing some value |
| that was not constant across each section of the group and so lead |
| to indeterminate results.</p> |
| <p>The <code>GROUP BY</code> clause can involve an expression. If the expression |
| is named, then the value is included in the columns, before |
| projection. An unnamed expression is used for grouping but the |
| value is not placed in the result set formed by the <code>GROUP BY</code> |
| clause.</p> |
| <pre><code>SELECT ?productId ?cost |
| { . . . |
| } |
| GROUP BY ?productId (?num * ?price AS ?cost) |
| </code></pre> |
| <h2 id="having">HAVING</h2> |
| <p>A query may specify a HAVING clause to apply a filter to the result |
| set after grouping. The filter may involve variables from the |
| <code>GROUP BY</code> clause or aggregations.</p> |
| <pre><code>SELECT ?p ?q |
| { . . . |
| } |
| GROUP BY ?p ?q |
| HAVING (count(distinct *) > 1) |
| </code></pre> |
| <h2 id="aggregation">Aggregation</h2> |
| <p>Currently supported aggregations:</p> |
| <table> |
| <thead> |
| <tr> |
| <th>Aggregator</th> |
| <th>Description</th> |
| </tr> |
| </thead> |
| <tbody> |
| <tr> |
| <td><code>count(*)</code></td> |
| <td>Count rows of each group element, or the whole result set if no <code>GROUP BY</code>.</td> |
| </tr> |
| <tr> |
| <td><code>count(distinct *)</code></td> |
| <td>Count the distinct rows of each group element, or the whole result set if no <code>GROUP BY</code>.</td> |
| </tr> |
| <tr> |
| <td><code>count(?var)</code></td> |
| <td>Count the number of times <code>?var</code> is bound in a group.</td> |
| </tr> |
| <tr> |
| <td><code>count(distinct ?var)</code></td> |
| <td>Count the number of distinct values <code>?var</code> is bound to in a group.</td> |
| </tr> |
| <tr> |
| <td><code>sum(?x)</code></td> |
| <td>Sum the variable over the group (non-numeric values and unbound values are ignored).</td> |
| </tr> |
| </tbody> |
| </table> |
| <p>When a variable is used, what is being counted is occurrences of |
| RDF terms, that is names. It is not a count of individuals because |
| two names can refer to the same individual.</p> |
| <p>If there was no explicit <code>GROUP BY</code> clause, then it is as if the |
| whole of the result set forms a single group element. |
| Equivalently, it is <code>GROUP BY</code> of no variables. Only aggregation |
| expressions make sense in the SELECT clause as there are no |
| variables from the query pattern to project out.</p> |
| <p><a href="index.html">ARQ documentation index</a></p> |
| |
| </article> |
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| <nav id="TableOfContents"> |
| <ul> |
| <li><a href="#group-by">GROUP BY</a></li> |
| <li><a href="#having">HAVING</a></li> |
| <li><a href="#aggregation">Aggregation</a></li> |
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