query in xml

Hi !
I'm using soap to make some query into an old legacy system (written
in c and c++). We would like to have one (and only one) soap method
called 'query' with one parameter (string query). The idea we have is
to implement and XML query language. Queries will be carried by soap.
I look Xquery documents, but this language is for queries into XML
documents.
Does anyone knows some documents about XML query language ?

Thanks !