# 22. Virtuoso Functions Guide - xpath_eval

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XPATH & XQUERY
Functions Index

## xpath_eval

Applies an XPATH expression to a context node and returns result(s).
xpath_eval (in xpath_expression varchar, in xml_tree XML Entity, [in index integer], [in named_params vector]);
Description

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.

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.

The expression can be passed parameters by specifying a fourth argument to xpath_eval(). 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.

Parameters
xpath_expression – A valid XPATH expression. In almost all application this is a string. There is a tricky extension that is used by BPEL-like applications: xpath_expression 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.
xml_tree – An XML entity such as that returned from the xtree_doc() function.
index – 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.
named_params – A vector of keyword/value parameters to be passed to the XPath processor.
Return Types

This function will return the first or index selected result of applying the XPath expression to the xml_tree input. If an index value of 0 is supplied then the output is an array.

Errors

SQL State | Error Code | Error Text | Description
--- | --- | --- | ---
XP370 | XI009 | Argument 2 of xpath_eval must be an entity, not a value of type [type name] ([type code]) | The xml_tree 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).

Examples
Finding the Authors of Document Titles

    select xt_file, xpath_eval ('//author', t) from xml_text
    	where xpath_contains (xt_text, '//chapter/title[. = 'Introduction']', t);

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.

xpath_eval and Repeating Nodes.

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.

    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;

Now we make a query that will return a vector of results, each vector element corresponding to a node-set of the result.

    SELECT xpath_eval('//beatle/@instrument', xml_tree_doc (article_xml), 0)
    	AS beatle_instrument FROM t_articles WHERE article_id = 2;

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.

Otherwise, we can select the first node-set by supplying 1 as the third parameter to xpath_eval:

    SELECT xpath_eval('//beatle/@instrument', xml_tree_doc (article_xml), 1)
    	AS beatle_instrument FROM t_articles WHERE article_id = 2;

Passing a named parameter to the XPath

    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>

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.

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.

See Also

xquery_eval()

xpath_contains SQL predicate

xcontains SQL Predicate