Syntax
------

EXTRACTVALUE(xml_frag, xpath_expr)

Description
-----------

The EXTRACTVALUE() function takes two string arguments: a fragment of XML
markup and an XPath expression, (also known as a locator). It returns the text
(That is, CDDATA), of the first text node which is a child of the element or
elements matching the XPath expression.

In cases where a valid XPath expression does not match any text nodes in a
valid XML fragment, (including the implicit /text() expression), the
EXTRACTVALUE() function returns an empty string.

Invalid Arguments
-----------------

When either the XML fragment or the XPath expression is NULL, the
EXTRACTVALUE() function returns NULL. When the XML fragment is invalid, it
raises a warning Code 1525:

Warning (Code 1525): Incorrect XML value: 'parse error at line 1 pos 11:
unexpected END-OF-INPUT'

When the XPath value is invalid, it generates an Error 1105:

ERROR 1105 (HY000): XPATH syntax error: ')'

Explicit text() Expressions
---------------------------

This function is the equivalent of performing a match using the XPath
expression after appending /text(). In other words:

SELECT
 EXTRACTVALUE('<cases><case>example</case></cases>', '/cases/case')
  AS 'Base Example',
 EXTRACTVALUE('<cases><case>example</case></cases>', '/cases/case/text()')
  AS 'text() Example';
+--------------+----------------+
| Base Example | text() Example |
+--------------+----------------+
| example      | example        |
+--------------+----------------+

Count Matches
-------------

When EXTRACTVALUE() returns multiple matches, it returns the content of the
first child text node of each matching element, in the matched order, as a
single, space-delimited string.

By design, the EXTRACTVALUE() function makes no distinction between a match on
an empty element and no match at all. If you need to determine whether no
matching element was found in the XML fragment or if an element was found that
contained no child text nodes, use the XPath count() function.

For instance, when looking for a value that exists, but contains no child text
nodes, you would get a count of the number of matching instances:

SELECT
 EXTRACTVALUE('<cases><case/></cases>', '/cases/case')
  AS 'Empty Example',
 EXTRACTVALUE('<cases><case/></cases>', 'count(/cases/case)')
  AS 'count() Example';
+---------------+-----------------+
| Empty Example | count() Example |
+---------------+-----------------+
|               |               1 |
+---------------+-----------------+

Alternatively, when looking for a value that doesn't exist, count() returns 0.

SELECT
 EXTRACTVALUE('<cases><case/></cases>', '/cases/person')
  AS 'No Match Example',
 EXTRACTVALUE('<cases><case/></cases>', 'count(/cases/person)')
  AS 'count() Example';
+------------------+-----------------+
| No Match Example | count() Example |
+------------------+-----------------+
|                  |                0|
+------------------+-----------------+

Matches
-------

Important: The EXTRACTVALUE() function only returns CDDATA. It does not return
tags that the element might contain or the text that these child elements
contain.

SELECT

EXTRACTVALUE('<cases><case>Person<email>x@example.com</email></case></cases>',
'/cases')
 AS Case;
+--------+
| Case   |
+--------+
| Person |
+--------+

Note, in the above example, while the XPath expression matches to the parent
<case> instance, it does not return the contained <email> tag or its content.

Examples
--------

SELECT
  ExtractValue('<a>ccc<b>ddd</b></a>', '/a')            AS val1,
  ExtractValue('<a>ccc<b>ddd</b></a>', '/a/b')          AS val2,
  ExtractValue('<a>ccc<b>ddd</b></a>', '//b')           AS val3,
  ExtractValue('<a>ccc<b>ddd</b></a>', '/b')            AS val4,
  ExtractValue('<a>ccc<b>ddd</b><b>eee</b></a>', '//b') AS val5;
+------+------+------+------+---------+
| val1 | val2 | val3 | val4 | val5    |
+------+------+------+------+---------+
| ccc  | ddd  | ddd  |      | ddd eee |
+------+------+------+------+---------+

URL: https://mariadb.com/kb/en/extractvalue/