Where can I learn about the efforts to create an XML-based query language?

Alessandro A. Garbagnati

There are two different XML based query languages. One is the W3C language XML-QL (http://www.w3.org/TR/1998/NOTE-xml-ql-19980819/.
The second one is XQL (http://www.ibiblio.org/xql/).
In addition, there are few different opinions if XPath (http://www.w3.org/TR/xpath) can be considered, or not, an XML Query language. This is mainly based on the fact that being "[...] a language for addressing parts of an XML document. [...]", it does much of what a query language should (and would) do.

An interesting reading, anyway, is this article (from the W3Consortium): Ten Features Necessary for an XML Query Language.