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<h1>22. Virtuoso Functions Guide - xquery_eval</h1>
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<h2>xquery_eval</h2>
<div class="refpurpose">Applies an XQUERY expression to a context node and returns result(s).</div>
<div class="funcsynopsis">
<span class="funcdef">
<span class="function">xquery_eval</span>
</span>
(<span class="paramdef">in <span class="parameter">xquery_expression</span> varchar</span>,
<span class="paramdef">in <span class="parameter">xml_tree</span> XML Entity</span>,
<span class="paramdefoptional">[in <span class="optional">index</span> integer]</span>,
<span class="paramdefoptional">[in <span class="optional">named_params</span> vector]</span>);
</div>
<div class="refsect1">
<div class="refsect1title">Description</div>
<p>
The xquery_eval function returns the result of applying the xquery expression to the
context node. By default only the first result is returned, but supplying a third argument
allows you to specify an index for the value, the default assumes a value of 1 here.
A value of 0 returns an array of 0 or more elements, one for each value calculated by the
xquery expression.
</p>
<p>
When an entity is returned in a result set to a client the client will see an nvarchar value containing the
serialization of the entity, complete with markup.
When the entity is passed as a SQL value it remains
an entity referencing the node of a parsed XML tree, permitting navigation inside the tree.
</p>
<p>
The expression can be passed parameters by specifying a fourth argument
to <span class="computeroutput">xquery_eval()</span>. This will be a vector of name/value
pairs. The values can be referenced from inside XPath expression by using
their names with '$' prefix. You may use any Virtuoso data type.
The names in the parameter vector should appear without the '$' sign.
If any of the parameter values is NULL the parameter will be ignored because NULL
has no XPath counterpart.
If the same name appears more than once in the vector, the last name/value pair is
used and all preceding pairs with this name are silently ignored.
Obviously, names should be strings that are valid XQuery variable names.
</p>
</div>
<div class="refsect1">
<div class="refsect1title">Parameters</div>
<div class="refsect2">
<span class="refsect2title">xquery_expression – </span>
A valid XQuery expression. In almost all applications this is a string.
There is a tricky extension that is used by BPEL-like applications: <span class="computeroutput">xpath_expression</span> can be an XML entity
whost string-value is a valid XQuery expression. An example of such an entity is "select" or "test"
attribute in XSLT stylesheet. This trick lets XQuery processor to resolve namespace prefixes by looking at namespace declarations
at the header of the stylesheet.</div>
<div class="refsect2">
<span class="refsect2title">xml_tree – </span>
An XML entity such as that returned from the <span class="computeroutput">xtree_doc()</span> function.</div>
<div class="refsect2">
<span class="refsect2title">index – </span>
Result index. This parameter is optional. If omitted a value of 1 is assumed,
meaning only the first result is returned. If a value of 0 is supplied then an array of 0 or more
results is returned containing one element per result. (Note that results can be in turn sequences of atomic values).</div>
<div class="refsect2">
<span class="refsect2title">named_params – </span>
A vector of keyword/value parameters to be passed to the
XQuery processor.</div>
</div>
<div class="refsect1">
<div class="refsect1title">Return Types</div>
<p>This function will return the
first or index selected result of applying the xpath expression to the <span class="computeroutput">xml_tree</span> input.
If an index value of 0 is supplied then the output is an array.</p>
</div>
<div class="refsect1">
<div class="refsect1title">Examples</div>
<a name="ex_xquery_eval" />
<div class="example">
<div class="exampletitle">Finding the Authors of Document Titles</div>
<div>
<pre class="programlisting">
select xt_file, xquery_eval ('<authors>//author</authors>', t) from xml_text
where xpath_contains (xt_text, '//chapter/title[. = 'Introduction']', t);
</pre>
</div>
<p>
This will select all titles that are descendants of chapter and have a string value of
'Introduction'. This will next evaluate //author in the context of each, retrieving the
author entities in the document of the title and construct an element whose name is "authors"
and list of children contains all retrieved entities.
</p>
</div>
</div>
<div class="refsect1">
<div class="refsect1title">See Also</div>
<p>
<a href="fn_xpath_eval.html">xpath_eval()</a>
</p>
<p>
<a href="queryingxmldata.html#xpathcontainsSQLPred">xpath_contains SQL predicate</a>
</p>
<p>
<a href="queryingxmldata.html#xcontainspredicate">xcontains SQL predicate</a>
</p>
<p>
<a href="xpf_processXQuery.html">processXQuery()</a>
</p>
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