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| <h1 class="title">ARQ - Assignment</h1> |
| |
| <p>ARQ includes support for a logical assignment of variables. If the variable is |
| already bound, it acts like a filter, otherwise the value is assignment. |
| This makes it position independent.</p> |
| <p>This involves is syntactic extension and is available is the query |
| is parsed with language <code>Syntax.syntaxARQ</code> (which is the default).</p> |
| <p>See also <a href="select_expr.html">SELECT expressions</a> which is also a |
| form of assignment.</p> |
| <h2 id="assignment">Assignment</h2> |
| <p>The general form is:</p> |
| <pre><code>LET ( variable := expression ) |
| </code></pre> |
| <p>For example:</p> |
| <pre><code>LET ( ?x := 2 ) |
| |
| { ?x :name ?name . |
| LET ( ?age2 := ?age - 21 ) |
| </code></pre> |
| <p>Note: Assignment is “<strong>:=</strong>”</p> |
| <h2 id="assignment-rules">Assignment Rules</h2> |
| <p>ARQ assignment is single assignment, that is, once a variable is |
| assigned a binding, then it can not be changed in the same query |
| solution.</p> |
| <p>Only one <code>LET</code> expression per variable is allowed in a single |
| scope.</p> |
| <p>The execution rules are:</p> |
| <ul> |
| <li>If the expression does not evaluate (e.g. unbound variable in |
| the expression), no assignment occurs and the query continues.</li> |
| <li>If the variable is unbound, and the expression evaluates, the |
| variable is bound to the value.</li> |
| <li>If the variable is bound to the same value as the expression |
| evaluates, nothing happens and the query continues.</li> |
| <li>If the variable is bound to a different value as the expression |
| evaluates, an error occurs and the current solution will be |
| excluded from the results.</li> |
| </ul> |
| <p>Note that “same value” means the same as applies to graph pattern |
| matching, not to FILTER expressions. Some graph implementation only |
| provide same-term graph pattern matching. FILTERs always do |
| value-based comparisons for “=” for all graphs.</p> |
| <h2 id="use-with-construct">Use with CONSTRUCT</h2> |
| <p>One use is to perform some calculation prior to forming the result |
| graph in a CONSTRUCT query.</p> |
| <pre><code>CONSTRUCT { ?x :lengthInCM ?cm } |
| WHERE |
| { |
| ?x :lengthInInches ?inch . |
| LET ( ?cm := ?inches/2.54 ) |
| } |
| </code></pre> |
| <h2 id="use-with-bound">Use with !BOUND</h2> |
| <p>The OPTIONAL/!BOUND/FILTER idiom for performing limited negation of |
| a pattern in SPARQL can be inconvenient because it requires a |
| variable in the OPTIONAL to be assigned by pattern matching. Using |
| a LET can make that easier; here, we assign to ?z (any value will |
| do) to mark when the matching pattern included the OPTIONAL |
| pattern.</p> |
| <p>Example: ?x with no “:p 1” triple:</p> |
| <pre><code>{ |
| ?x a :FOO . |
| OPTIONAL { ?x :p 1 . LET (?z := true) } |
| FILTER ( !BOUND(?z) ) |
| } |
| </code></pre> |
| <p>Note that <a href="negation.html">negation is supported properly</a> through |
| the <code>NOT EXISTS</code> form.</p> |
| <p><a href="index.html">ARQ documentation index</a></p> |
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