User variables (weak checking).  Variables using the syntax $@variable_name (that is, user variables) are not checked. No warnings or errors are issued by the server if a variable has the wrong type or has previously not been assigned a value. This also means the user is fully responsible for any typographical errors, since no warnings will be given if (for example) $@myvairable is used where $@myvariable was intended.

Example. 

mysql> SET @xml = '<a><b>X</b><b>Y</b></a>';
Query OK, 0 rows affected (0.00 sec)

mysql> SET @i =1, @j = 2;
Query OK, 0 rows affected (0.00 sec)

mysql> SELECT @i, ExtractValue(@xml, '//b[$@i]');
+------+--------------------------------+
| @i   | ExtractValue(@xml, '//b[$@i]') |
+------+--------------------------------+
|    1 | X                              |
+------+--------------------------------+
1 row in set (0.00 sec)

mysql> SELECT @j, ExtractValue(@xml, '//b[$@j]');
+------+--------------------------------+
| @j   | ExtractValue(@xml, '//b[$@j]') |
+------+--------------------------------+
|    2 | Y                              |
+------+--------------------------------+
1 row in set (0.00 sec)

mysql> SELECT @k, ExtractValue(@xml, '//b[$@k]');
+------+--------------------------------+
| @k   | ExtractValue(@xml, '//b[$@k]') |
+------+--------------------------------+
| NULL |                                |
+------+--------------------------------+
1 row in set (0.00 sec)

Variables in stored programs (strong checking).  Variables using the syntax $variable_name can be declared and used with these functions when they are called inside stored programs. Such variables are local to the stored program in which they are defined, and are strongly checked for type and value.

Example. 

mysql> DELIMITER |

mysql> CREATE PROCEDURE myproc ()
    -> BEGIN
    ->   DECLARE i INT DEFAULT 1;
    ->   DECLARE xml VARCHAR(25) DEFAULT '<a>X</a><a>Y</a><a>Z</a>';
    ->
    ->   WHILE i < 4 DO
    ->     SELECT xml, i, ExtractValue(xml, '//a[$i]');
    ->     SET i = i+1;
    ->   END WHILE;
    -> END |
Query OK, 0 rows affected (0.01 sec)

mysql> DELIMITER ;

mysql> CALL myproc;
+--------------------------+---+------------------------------+
| xml                      | i | ExtractValue(xml, '//a[$i]') |
+--------------------------+---+------------------------------+
| <a>X</a><a>Y</a><a>Z</a> | 1 | X                            |
+--------------------------+---+------------------------------+
1 row in set (0.00 sec)

+--------------------------+---+------------------------------+
| xml                      | i | ExtractValue(xml, '//a[$i]') |
+--------------------------+---+------------------------------+
| <a>X</a><a>Y</a><a>Z</a> | 2 | Y                            |
+--------------------------+---+------------------------------+
1 row in set (0.01 sec)

+--------------------------+---+------------------------------+
| xml                      | i | ExtractValue(xml, '//a[$i]') |
+--------------------------+---+------------------------------+
| <a>X</a><a>Y</a><a>Z</a> | 3 | Z                            |
+--------------------------+---+------------------------------+
1 row in set (0.01 sec)

Parameters.  Variables used in XPath expressions inside stored routines that are passed in as parameters are also subject to strong checking.

ExtractValue(xml_frag, xpath_expr)

ExtractValue() takes two string arguments, a fragment of XML markup xml_frag and an XPath expression xpath_expr (also known as a locator); it returns the text (CDATA) of the first text node which is a child of the element(s) matched by the XPath expression. It is the equivalent of performing a match using the xpath_expr after appending /text(). In other words, ExtractValue('<a><b>Sakila</b></a>', '/a/b') and ExtractValue('<a><b>Sakila</b></a>', '/a/b/text()') produce the same result.

If multiple matches are found, then the content of the first child text node of each matching element is returned (in the order matched) as a single, space-delimited string.

If no matching text node is found for the expression (including the implicit /text()) — for whatever reason, as long as xpath_expr is valid, and xml_frag consists of elements which are properly nested and closed — an empty string is returned. No distinction is made between a match on an empty element and no match at all. This is by design.

If you need to determine whether no matching element was found in xml_frag or such an element was found but contained no child text nodes, you should test the result of an expression that uses the XPath count() function. For example, both of these statements return an empty string, as shown here:

mysql> SELECT ExtractValue('<a><b/></a>', '/a/b');
+-------------------------------------+
| ExtractValue('<a><b/></a>', '/a/b') |
+-------------------------------------+
|                                     |
+-------------------------------------+
1 row in set (0.00 sec)

mysql> SELECT ExtractValue('<a><c/></a>', '/a/b');
+-------------------------------------+
| ExtractValue('<a><c/></a>', '/a/b') |
+-------------------------------------+
|                                     |
+-------------------------------------+
1 row in set (0.00 sec)

However, you can determine whether there was actually a matching element using the following:

mysql> SELECT ExtractValue('<a><b/></a>', 'count(/a/b)');
+-------------------------------------+
| ExtractValue('<a><b/></a>', 'count(/a/b)') |
+-------------------------------------+
| 1                                   |
+-------------------------------------+
1 row in set (0.00 sec)

mysql> SELECT ExtractValue('<a><c/></a>', 'count(/a/b)');
+-------------------------------------+
| ExtractValue('<a><c/></a>', 'count(/a/b)') |
+-------------------------------------+
| 0                                   |
+-------------------------------------+
1 row in set (0.01 sec)

mysql> 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 |
+------+------+------+------+---------+

Beginning with MySQL 5.1.8, this function uses the current SQL collation for making comparisons with contains(), performing the same collation aggregation as other string functions (such as CONCAT()), in taking into account the collation coercibility of their arguments; see Section 9.1.7.5, “Special Cases Where Collation Determination Is Tricky”, for an explanation of the rules governing this behavior.

(Previously, binary — that is, case-sensitive — comparison was always used.)

Beginning with MySQL 5.1.12, NULL is returned if xml_frag contains elements which are not properly nested or closed, and a warning is generated, as shown in this example:

mysql> SELECT ExtractValue('<a>c</a><b', '//a');
+-----------------------------------+
| ExtractValue('<a>c</a><b', '//a') |
+-----------------------------------+
| NULL                              |
+-----------------------------------+
1 row in set, 1 warning (0.00 sec)

mysql> SHOW WARNINGS;
+---------+------+-------------------------------------------------------------------------------------------+
| Level   | Code | Message                                                                                   |
+---------+------+-------------------------------------------------------------------------------------------+
| Warning | 1523 | Incorrect XML value: 'parse error at line 1 pos 11: END-OF-INPUT unexpected ('>' wanted)' |
+---------+------+-------------------------------------------------------------------------------------------+
1 row in set (0.00 sec)

mysql> SELECT ExtractValue('<a>c</a><b/>', '//a');
+-------------------------------------+
| ExtractValue('<a>c</a><b/>', '//a') |
+-------------------------------------+
| c                                   |
+-------------------------------------+
1 row in set (0.00 sec)

Prior to MySQL 5.1.12, an empty string was returned in such cases. (Bug#18201)

UpdateXML(xml_target, xpath_expr, new_xml)

This function replaces a single portion of a given fragment of XML markup xml_target with a new XML fragment new_xml, and then returns the changed XML. The portion of xml_target that is replaced matches an XPath expression xpath_expr supplied by the user. If no expression matching xpath_expr is found, or if multiple matches are found, the function returns the original xml_target XML fragment. All three arguments should be strings.

mysql> SELECT
    ->   UpdateXML('<a><b>ccc</b><d></d></a>', '/a', '<e>fff</e>') AS val1,
    ->   UpdateXML('<a><b>ccc</b><d></d></a>', '/b', '<e>fff</e>') AS val2,
    ->   UpdateXML('<a><b>ccc</b><d></d></a>', '//b', '<e>fff</e>') AS val3,
    ->   UpdateXML('<a><b>ccc</b><d></d></a>', '/a/d', '<e>fff</e>') AS val4,
    ->   UpdateXML('<a><d></d><b>ccc</b><d></d></a>', '/a/d', '<e>fff</e>') AS val5
    -> \G

*************************** 1. row ***************************
val1: <e>fff</e>
val2: <a><b>ccc</b><d></d></a>
val3: <a><e>fff</e><d></d></a>
val4: <a><b>ccc</b><e>fff</e></a>
val5: <a><d></d><b>ccc</b><d></d></a>

/tag

Matches <tag/> if and only if <tag/> is the root element.

Example: /a has a match in <a><b/></a> because it matches the outermost (root) tag. It does not match the inner a element in <b><a/></b> because in this instance it is the child of another element.

/tag1/tag2

Matches <tag2/> if and only if it is a child of <tag1/>, and <tag1/> is the root element.

Example: /a/b matches the b element in the XML fragment <a><b/></a> because it is a child of the root element a. It does not have a match in <b><a/></b> because in this case, b is the root element (and hence the child of no other element). Nor does the XPath expression have a match in <a><c><b/></c></a>; here, b is a descendant of a, but not actually a child of a.

This construct is extendable to three or more elements. For example, the XPath expression /a/b/c matches the c element in the fragment <a><b><c/></b></a>.

//tag

Matches any instance of <tag>.

Example: //a matches the a element in any of the following: <a><b><c/></b></a>; <c><a><b/></a></b>; <c><b><a/></b></c>.

// can be combined with /. For example, //a/b matches the b element in either of the fragments <a><b/></a> or <a><b><c/></b></a>

mysql> SET @xml = '<a><b><c>w</c><b>x</b><d>y</d>z</b></a>';
Query OK, 0 rows affected (0.00 sec)

mysql> SELECT @xml;
+-----------------------------------------+
| @xml                                    |
+-----------------------------------------+
| <a><b><c>w</c><b>x</b><d>y</d>z</b></a> |
+-----------------------------------------+
1 row in set (0.00 sec)

mysql> SELECT ExtractValue(@xml, '//b[1]');
+------------------------------+
| ExtractValue(@xml, '//b[1]') |
+------------------------------+
| x z                          |
+------------------------------+
1 row in set (0.00 sec)

mysql> SELECT ExtractValue(@xml, '//b[2]');
+------------------------------+
| ExtractValue(@xml, '//b[2]') |
+------------------------------+
|                              |
+------------------------------+
1 row in set (0.01 sec)

mysql> SELECT ExtractValue(@xml, '/descendant-or-self::*/b[1]');
+---------------------------------------------------+
| ExtractValue(@xml, '/descendant-or-self::*/b[1]') |
+---------------------------------------------------+
| x z                                               |
+---------------------------------------------------+
1 row in set (0.06 sec)

mysql> SELECT ExtractValue(@xml, '/descendant-or-self::*/b[2]');
+---------------------------------------------------+
| ExtractValue(@xml, '/descendant-or-self::*/b[2]') |
+---------------------------------------------------+
|                                                   |
+---------------------------------------------------+
1 row in set (0.00 sec)


mysql> SELECT ExtractValue(@xml, '/descendant-or-self::b[1]');
+-------------------------------------------------+
| ExtractValue(@xml, '/descendant-or-self::b[1]') |
+-------------------------------------------------+
| z                                               |
+-------------------------------------------------+
1 row in set (0.00 sec)

mysql> SELECT ExtractValue(@xml, '/descendant-or-self::b[2]');
+-------------------------------------------------+
| ExtractValue(@xml, '/descendant-or-self::b[2]') |
+-------------------------------------------------+
| x                                               |
+-------------------------------------------------+
1 row in set (0.00 sec)

The * operator acts as a “wildcard” that matches any element. For example, the expression /*/b matches the b element in either of the XML fragments <a><b/></a> or <c><b/></c>. However, the expression does not produce a match in the fragment <b><a/></b> because b must be a child of some other element. The wildcard may be used in any position: The expression /*/b/* will match any child of a b element that is itself not the root element.

You can match any of several locators using the | (UNION) operator. For example, the expression //b|//c matches all b and c elements in the XML target.

It is also possible to match an element based on the value of one or more of its attributes. This done using the syntax tag[@attribute="value"]. For example, the expression //b[@id="idB"] matches the second b element in the fragment <a><b id="idA"/><c/><b id="idB"/></a>. To match against any element having attribute="value", use the XPath expression //*[attribute="value"].

To filter multiple attribute values, simply use multiple attribute-comparison clauses in succession. For example, the expression //b[@c="x"][@d="y"] matches the element <b c="x" d="y"/> occurring anywhere in a given XML fragment.

To find elements for which the same attribute matches any of several values, you can use multiple locators joined by the | operator. For example, to match all b elements whose c attributes have either of the values 23 or 17, use the expression //b[@c="23"]|//b[@c="17"]. You can also use the logical or operator for this purpose: //b[@c="23" or @c="17"].

Nodeset-to-nodeset comparison (such as '/a/b[@c=@d]') is not supported.

Prior to MySQL 5.1.14, equality and inequality (= and !=) were the only supported comparison operators. Beginning with MySQL 5.1.14, all of the standard XPath comparison operators are supported. (Bug#22823)

Relative locator expressions are resolved in the context of the root node. For example, consider the following query and result:

mysql> SELECT ExtractValue(
    ->   '<a><b c="1">X</b><b c="2">Y</b></a>',
    ->    'a/b'
    -> ) AS result;
+--------+
| result |
+--------+
| X Y    |
+--------+
1 row in set (0.03 sec)

In this case, the locator a/b resolves to /a/b.

Relative locators are also supported within predicates. In the following example, d[../@c="1"] is resolved as /a/b[@c="1"]/d:

mysql> SELECT ExtractValue(
    ->      '<a>
    ->        <b c="1"><d>X</d></b>
    ->        <b c="2"><d>X</d></b>
    ->      </a>',
    ->      'a/b/d[../@c="1"]')
    -> AS result;
+--------+
| result |
+--------+
| X      |
+--------+
1 row in set (0.00 sec)

Locators prefixed with expressions that evaluate as scalar values — including variable references, literals, numbers,and scalar function calls — are not supported. Beginning with MySQL 5.1.32, they are disallowed, and their use results in an error. (Bug#42495)

The :: operator is not supported in combination with node types such as the following:

However, name tests (such as axis::name and axis::*) are supported, as shown in these examples:

mysql> SELECT ExtractValue('<a><b>x</b><c>y</c></a>','/a/child::b');
+-------------------------------------------------------+
| ExtractValue('<a><b>x</b><c>y</c></a>','/a/child::b') |
+-------------------------------------------------------+
| x                                                     |
+-------------------------------------------------------+
1 row in set (0.02 sec)

mysql> SELECT ExtractValue('<a><b>x</b><c>y</c></a>','/a/child::*');
+-------------------------------------------------------+
| ExtractValue('<a><b>x</b><c>y</c></a>','/a/child::*') |
+-------------------------------------------------------+
| x y                                                   |
+-------------------------------------------------------+
1 row in set (0.01 sec)

“Up-and-down” navigation is not supported in cases where the path would lead “above” the root element. That is, you cannot use expressions which match on descendants of ancestors of a given element, where one or more of the ancestors of the current element is also an ancestor of the root element (see Bug#16321).

The following XPath functions are not supported, or have known issues as indicated:

The following axes are not supported:

Never accepted untested data from users in your application.

Check all user-submited data for type; reject or convert data that is of the wrong type

Test numerical data for out of range values; truncate, round, or reject values that are out of range. Test strings for illegal characters and either strip them out or reject input containing them.

Do not output explicit error messages that might provide an unauthorized user with clues that could be used to compromise the system; log these to a file or database table instead.

Always test outgoing data for type and allowable values.

Never allow unauthorized users to view error messages that might provide information about the application that could be used to exploit it.