This section discusses XML and related functionality in MySQL.
It is possible to obtain XML-formatted output from MySQL in the
mysql and mysqldump
clients by invoking them with the
--xml option. See
Section 4.5.1, “mysql — The MySQL Command-Line Client”, and Section 4.5.4, “mysqldump — A Database Backup Program”.. A useful resource for those new to XPath or who desire a refresher in the basics is the Zvon.org XPath Tutorial, which is available in several languages.
These functions remain under development. We continue to improve these and other aspects of XML and XPath functionality in MySQL 5.6 and onwards. You may discuss these, ask questions about them, and obtain help from other users with them in the MySQL XML User Forum.
XPath expressions used with these functions support user variables and local stored program variables. User variables are weakly checked; variables local to stored programs are strongly checked (see also Bug #26518):
User variables (weak checking). Variables using the syntax
$@(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 are given if (for example)
variable_name
$@myvairableis used where
$@myvariablewas intended.
Example:
mysql> SET @xml = '<a><b>X</b><b>Y</b></a>'; Query OK, 0 rows affected (0.00 sec) mysql> SET @i =1, @j = 2; Query OK, 0 rows affected (0.00 sec) mysql> SELECT @i, ExtractValue(@xml, '//b[$@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
$can be declared and used with these functions when they are called inside stored programs. Such variables are local to the stored program in which they are defined, and are strongly checked for type and value.
variable_name
Example:
mysql> DELIMITER | mysql> CREATE PROCEDURE myproc () -> BEGIN -> DECLARE i INT DEFAULT 1; -> DECLARE xml VARCHAR(25) DEFAULT '<a>X</a><a>Y</a><a>Z</a>'; -> -> WHILE i < 4 DO -> SELECT xml, i, ExtractValue(xml, '//a[$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.
Expressions containing user variables or variables local to stored programs must otherwise (except for notation) conform to the rules for XPath expressions containing variables as given in the XPath 1.0 specification.
A user variable used to store an XPath expression is treated as an empty string. Because of this, it is not possible to store an XPath expression as a user variable. (Bug #32911)
ExtractValue(
xml_frag,
xpath_expr)
ExtractValue()takes two string arguments, a fragment of XML markup
xml_fragand 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 or elements matched by the XPath expression.
Using this function is the equivalent of performing a match using the
xpath_exprafter 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, 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_expris valid, and
xml_fragconsists 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_fragor 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)Note
In MySQL 5.6.28 and MySQL 5.6.29, when
ExtractValue()failed to find a match for the supplied expression, it returned
NULL. This issue was resolved in MySQL 5.6.30. (Bug #22552615)
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)Important
ExtractValue()returns only
CDATA, and does not return any tags that might be contained within a matching tag, nor any of their content (see the result returned as
val1in the following example).
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 | +------+------+------+------+---------+
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 10.8.4, “Collation Coercibility in Expressions”, for an explanation of the rules governing this behavior.
(Previously, binary—that is, case-sensitive—comparison was always used.)
NULLis returned if
xml_fragcontains elements which are not properly nested or closed,)
UpdateXML(
xml_target,
xpath_expr,
new_xml)
This function replaces a single portion of a given fragment of XML markup
xml_targetwith a new XML fragment
new_xml, and then returns the changed XML. The portion of
xml_targetthat is replaced matches an XPath expression
xpath_exprsupplied by the user.
If no expression matching
xpath_expris found, or if multiple matches are found, the function returns the original
xml_targetXML 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>
A discussion in depth of XPath syntax and usage are beyond the scope of this manual. Please see the XML Path Language (XPath) 1.0 specification for definitive information. A useful resource for those new to XPath or who are wishing a refresher in the basics is the Zvon.org XPath Tutorial, which is available in several languages.
Descriptions and examples of some basic XPath expressions follow:
/
tag
Matches
<if and only if
tag/>
<is the root element.
tag/>
Example:
/ahas a match in
<a><b/></a>because it matches the outermost (root) tag. It does not match the inner
aelement in
<b><a/></b>because in this instance it is the child of another element.
/
tag1/
tag2
Matches
<if and only if it is a child of
tag2/>
<, and
tag1/>
<is the root element.
tag1/>
Example:
/a/bmatches the
belement 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,
bis 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,
bis 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/cmatches the
celement in the fragment
<a><b><c/></b></a>.
//
tag
Matches any instance of
<.
tag>
Example:
//amatches the
aelement 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/bmatches the
belement in either of the fragments
<a><b/></a>or
<c><a><b/></a></c>.Note
//is the equivalent of
tag
/descendant-or-self::*/. A common error is to confuse this with
tag
/descendant-or-self::, although the latter expression can actually lead to very different results, as can be seen here:
tag
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
/*/bmatches the
belement 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
bmust be a child of some other element. The wildcard may be used in any position: The expression
/*/b/*matches any child of a
belement that is itself not the root element.
You can match any of several locators using the
|(
UNION) operator. For example, the expression
//b|//cmatches all
band
celements 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
. For example, the expression
tag[@
attribute="
value"]
//b[@id="idB"]matches the second
belement in the fragment
<a><b id="idA"/><c/><b id="idB"/></a>. To match against any element having
, use the XPath expression
attribute="
value"
//*[.
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
belements whose
cattributes have either of the values 23 or 17, use the expression
//b[@c="23"]|//b[@c="17"]. You can also use the logical
oroperator for this purpose:
//b[@c="23" or @c="17"].Note
The difference between
orand
|is that
orjoins conditions, while
|joins result sets.
XPath Limitations. The XPath syntax supported by these functions is currently subject to the following limitations:
Nodeset-to-nodeset comparison (such as
'/a/b[@c=@d]') is not supported.
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/bresolves 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 permitted, and their use results in an error.
The
::operator is not supported in combination with node types such as the following:
axis::comment()
axis::text()
axis::processing-instructions()
axis::node()
However, name tests (such as
and
axis::
name
) are supported, as shown in these examples:
axis::*
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:
id()
lang()
local-name()
name()
namespace-uri()
normalize-space()
starts-with()
string()
substring-after()
substring-before()
translate()
The following axes are not supported:
following-sibling
following
preceding-sibling
preceding
XPath expressions passed as arguments to
ExtractValue() and
UpdateXML() may contain the colon
character (
:) in element selectors, which
enables their use with markup employing XML namespaces notation.
For example:
mysql> SET @xml = '<a>111<b:c>222<d>333</d><e:f>444</e:f></b:c></a>'; Query OK, 0 rows affected (0.00 sec) mysql> SELECT ExtractValue(@xml, '//e:f'); +-----------------------------+ | ExtractValue(@xml, '//e:f') | +-----------------------------+ | 444 | +-----------------------------+ 1 row in set (0.00 sec) mysql> SELECT UpdateXML(@xml, '//b:c', '<g:h>555</g:h>'); +--------------------------------------------+ | UpdateXML(@xml, '//b:c', '<g:h>555</g:h>') | +--------------------------------------------+ | <a>111<g:h>555</g:h></a> | +--------------------------------------------+ 1 row in set (0.00 sec)
This is similar in some respects to what is permitted by
Apache Xalan and
some other parsers, and is much simpler than requiring namespace
declarations or the use of the
namespace-uri()
and
local-name() functions.
Error handling.
For both
ExtractValue() and
UpdateXML(), the XPath locator
used must be valid and the XML to be searched must consist of
elements which are properly nested and closed. If the locator is
invalid, an error is generated:
mysql> SELECT ExtractValue('<a>c</a><b/>', '/&a'); ERROR 1105 (HY000): XPATH syntax error: '&a'
If
xml_frag does not consist of
elements which are properly nested and closed,
NULL is returned)
The replacement XML used as the third argument to
UpdateXML() is
not checked to determine whether it
consists solely of elements which are properly nested and
closed.
XPath Injection. code injection occurs when malicious code is introduced into the system to gain unauthorized access to privileges and data. It is based on exploiting assumptions made by developers about the type and content of data input from users. XPath is no exception in this regard.
A common scenario in which this can happen is the case of application which handles authorization by matching the combination of a login name and password with those found in an XML file, using an XPath expression like this one:
//user[login/text()='neapolitan' and password/text()='1c3cr34m']/attribute::id
This is the XPath equivalent of an SQL statement like this one:
SELECT id FROM users WHERE login='neapolitan' AND password='1c3cr34m';
A PHP application employing XPath might handle the login process like this:
<?php $file = "users.xml"; $login = $POST["login"]; $password = $POST["password"]; $xpath = "//user[login/text()=$login and password/text()=$password]/attribute::id"; if( file_exists($file) ) { $xml = simplexml_load_file($file); if($result = $xml->xpath($xpath)) echo "You are now logged in as user $result[0]."; else echo "Invalid login name or password."; } else exit("Failed to open $file."); ?>
No checks are performed on the input. This means that a malevolent
user can “short-circuit” the test by entering
' or 1=1 for both the login name and password,
resulting in
$xpath being evaluated as shown
here:
//user[login/text()='' or 1=1 and password/text()='' or 1=1]/attribute::id
Since the expression inside the square brackets always evaluates
as
true, it is effectively the same as this
one, which matches the
id attribute of every
user element in the XML document:
//user/attribute::id
One way in which this particular attack can be circumvented is
simply by quoting the variable names to be interpolated in the
definition of
$xpath, forcing the values passed
from a Web form to be converted to strings:
$xpath = "//user[login/text()='$login' and password/text()='$password']/attribute::id";
This is the same strategy that is often recommended for preventing SQL injection attacks. In general, the practices you should follow for preventing XPath injection attacks are the same as for preventing SQL injection:
Never accepted untested data from users in your application.
Check all user-submitted data for type; reject or convert data that is of the wrong type
Test numeric 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.
Just as SQL injection attacks can be used to obtain information about database schemas, so can XPath injection be used to traverse XML files to uncover their structure, as discussed in Amit Klein's paper Blind XPath Injection (PDF file, 46KB).
It is also important to check the output being sent back to the
client. Consider what can happen when we use the MySQL
ExtractValue() function:
mysql> SELECT ExtractValue( -> LOAD_FILE('users.xml'), -> '//user[login/text()="" or 1=1 and password/text()="" or 1=1]/attribute::id' -> ) AS id; +-------------------------------+ | id | +-------------------------------+ | 00327 13579 02403 42354 28570 | +-------------------------------+ 1 row in set (0.01 sec)
Because
ExtractValue() returns
multiple matches as a single space-delimited string, this
injection attack provides every valid ID contained within
users.xml to the user as a single row of
output. As an extra safeguard, you should also test output before
returning it to the user. Here is a simple example:
mysql> SELECT @id = ExtractValue( -> LOAD_FILE('users.xml'), -> '//user[login/text()="" or 1=1 and password/text()="" or 1=1]/attribute::id' -> ); Query OK, 0 rows affected (0.00 sec) mysql> SELECT IF( -> INSTR(@id, ' ') = 0, -> @id, -> 'Unable to retrieve user ID') -> AS singleID; +----------------------------+ | singleID | +----------------------------+ | Unable to retrieve user ID | +----------------------------+ 1 row in set (0.00 sec)
In general, the guidelines for returning data to users securely are the same as for accepting user input. These can be summed up as:
Always test outgoing data for type and permissible values.
Never permit unauthorized users to view error messages that might provide information about the application that could be used to exploit it.