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 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). 

