XQuery

A query language that uses the structure of XML intelligently can express queries across all these kinds of data, whether physically stored in XML or viewed as XML via middleware. This specification describes a query language called XQuery, which is designed to be broadly applicable across many types of XML data sources.

http://www.w3.org/TR/xquery/

http://www.xml.com/pub/a/2002/10/16/xquery.html

http://www-106.ibm.com/developerworks/xml/library/x-xquery.html