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<?xml version="1.0" encoding="ISO-8859-1"?>
<!--
-
- This file is part of the OpenLink Software Virtuoso Open-Source (VOS)
- project.
-
- Copyright (C) 1998-2018 OpenLink Software
-
- This project is free software; you can redistribute it and/or modify it
- under the terms of the GNU General Public License as published by the
- Free Software Foundation; only version 2 of the License, dated June 1991.
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- This program is distributed in the hope that it will be useful, but
- WITHOUT ANY WARRANTY; without even the implied warranty of
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- General Public License for more details.
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- You should have received a copy of the GNU General Public License along
- with this program; if not, write to the Free Software Foundation, Inc.,
- 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA
-
-
-->
<refentry id="fn_xpath_eval">
<refmeta>
<refentrytitle>xpath_eval</refentrytitle>
<refmiscinfo>xml</refmiscinfo>
</refmeta>
<refnamediv>
<refname>xpath_eval</refname>
<refpurpose>Applies an XPATH expression to a context node and returns result(s).</refpurpose>
</refnamediv>
<refsynopsisdiv>
<funcsynopsis id="fsyn_xpath_eval">
<funcprototype id="fproto_xpath_eval">
<funcdef><function>xpath_eval</function></funcdef>
<paramdef>in <parameter>xpath_expression </parameter>varchar</paramdef>
<paramdef>in <parameter>xml_tree </parameter>XML Entity</paramdef>
<paramdef><optional>in <parameter>index </parameter>integer</optional></paramdef>
<paramdef><optional>in <parameter>named_params</parameter> vector</optional></paramdef>
</funcprototype>
</funcsynopsis>
</refsynopsisdiv>
<refsect1 id="desc_xpath_eval">
<title>Description</title>
<para>
This function returns the result of applying the XPath
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
selected by the XPath expression.
</para>
<para>
When this function returns an entity in a result set, the client will
see an nvarchar value containing the serialization of the entity,
complete with markup. When the entity is passed as an SQL value it
remains an entity referencing the node of a parsed XML tree,
permitting navigation inside the tree.
</para>
<para>
The expression can be passed parameters by specifying a fourth argument
to <function>xpath_eval()</function>. 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 XPath variable names.
</para>
</refsect1>
<refsect1 id="params_xpath_eval">
<title>Parameters</title>
<refsect2><title>xpath_expression</title>
<para>A valid XPATH expression. In almost all application this is a string.
There is a tricky extension that is used by BPEL-like applications: <parameter>xpath_expression</parameter> can be an XML entity
whost string-value is a valid XPATH expression. An example of such an entity is "select" or "test"
attribute in XSLT stylesheet. This trick lets XPATH processor to resolve namespace prefixes by looking at namespace declarations
at the header of the stylesheet.</para>
</refsect2>
<refsect2><title>xml_tree</title>
<para>An XML entity such as that returned from the <function>xtree_doc()</function> function.</para>
</refsect2>
<refsect2><title>index</title>
<para>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 elements is
returned containing one element per result.</para>
</refsect2>
<refsect2><title>named_params</title>
<para>A vector of keyword/value parameters to be passed to the
XPath processor.</para>
</refsect2>
</refsect1>
<refsect1 id="ret_xpath_eval">
<title>Return Types</title>
<para>This function will return the first or
<parameter>index</parameter> selected result of applying the XPath
expression to the <parameter>xml_tree</parameter> input. If an <parameter>index</parameter>
value of 0 is supplied then the output is an array.</para>
</refsect1>
<refsect1 id="errors_xpath_eval">
<title>Errors</title>
<table>
<title>Errors signalled by xpath_eval</title>
<tgroup cols="4">
<thead>
<row><entry>SQL State</entry><entry>Error Code</entry><entry>Error Text</entry><entry>Description</entry>
</row>
</thead>
<tbody>
<row>
<entry>
<errorcode>XP370</errorcode>
</entry>
<entry>
<errorcode>XI009</errorcode>
</entry>
<entry>
<errorname>Argument 2 of xpath_eval must be an entity, not a value of type [type name] ([type code])</errorname>
</entry>
<entry>The <parameter>xml_tree</parameter> parameter should always be an entity.
In some cases the XPath expression does not use context entity, but it should be provided anyway
(e.g. some fake document).</entry>
</row>
</tbody>
</tgroup>
</table>
</refsect1>
<refsect1 id="examples_xpath_eval">
<title>Examples</title>
<example id="ex_xpath_eval">
<title>Finding the Authors of Document Titles</title>
<programlisting>
select xt_file, xpath_eval ('//author', t) from xml_text
where xpath_contains (xt_text, '//chapter/title[. = 'Introduction']', t);
</programlisting>
<para>
This will select all titles that are descendants of chapter and have a
string value of 'Introduction'. This will next evaluate
<parameter>//author</parameter> in the context of each, retrieving the
author entities in the document of the title.
</para>
</example>
<example id="ex_xpath_eval2">
<title>xpath_eval and Repeating Nodes.</title>
<para>This example shows how the xpath_eval can be used to retrieve
multiple node answers to queries. First to set the scene consider the following
statements that create a table with XML inside.</para>
<programlisting><![CDATA[
CREATE TABLE t_articles (
article_id int NOT NULL,
article_title varchar(255) NOT NULL,
article_xml long varchar
);
insert into t_articles (article_id, article_title) values (1, 'a');
insert into t_articles (article_id, article_title) values (2, 'b');
UPDATE t_articles SET article_xml = '
<beatles id = "b1">
<beatle instrument = "guitar" alive = "no">john lennon</beatle>
<beatle instrument = "guitar" alive = "no">george harrison</beatle>
</beatles>'
WHERE article_id = 1;
UPDATE t_articles SET article_xml = '
<beatles id = "b2">
<beatle instrument = "bass" alive = "yes">paul mccartney</beatle>
<beatle instrument = "drums" alive = "yes">ringo starr</beatle>
</beatles>'
WHERE article_id = 2;
]]></programlisting>
<para>Now we make a query that will return a vector of results, each
vector element corresponding to a node-set of the result.</para>
<programlisting><![CDATA[
SELECT xpath_eval('//beatle/@instrument', xml_tree_doc (article_xml), 0)
AS beatle_instrument FROM t_articles WHERE article_id = 2;
]]></programlisting>
<para>The repeating nodes are returned as part of a vector, the third argument
to xpath_eval is set to 0, which means that it is to return all nodes.</para>
<para>Otherwise, we can select the first node-set by supplying 1 as the third
parameter to xpath_eval: </para>
<programlisting><![CDATA[
SELECT xpath_eval('//beatle/@instrument', xml_tree_doc (article_xml), 1)
AS beatle_instrument FROM t_articles WHERE article_id = 2;
]]></programlisting>
</example>
<example id="ex_xpath_eval3">
<title>Passing a named parameter to the XPath</title>
<programlisting><![CDATA[
SQL> select xpath_eval(
'//phone[../name like $$n]',
xtree_doc ('<phonebook>
<dept><name>Installation Service</name><phone>555-1111</phone></dept>
<dept><name>Tech Support</name><phone>555-1112</phone></dept>
</phonebook>'),
1,
vector('n', 'Tech%') );
callret
VARCHAR
_______________________________________________________________________________
<phone>555-1112</phone>
]]></programlisting>
<para>
Like in SQL queries, XPath parameters are used to avoid printing literal values into
the text of the query. The listing above demonstrates how to find the phone of
the first department whose name is like the specified pattern.
</para>
<para>
Note that if the text of the XPath expression is entered as a string in ISQL,
dollar signs in front of parameter names should be doubled, as in this sample.
ISQL uses the same syntax ($name) for macro expansion, and double dollar sign
is replaced by ISQL with single dollar sign character.
</para>
</example>
</refsect1>
<refsect1 id="seealso_xpath_eval">
<title>See Also</title>
<para><link linkend="fn_xquery_eval"><function>xquery_eval()</function></link></para>
<para><link linkend="xpathcontainsSQLPred">xpath_contains SQL predicate</link></para>
<para><link linkend="xcontainspredicate">xcontains SQL Predicate</link></para>
<!-- <para><link linkend="fn_xpath_explain"><function>xpath_explain()</function></link></para> -->
</refsect1>
</refentry>
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