1. A system enabling the querying of data modeled by an XML schema, comprising: at least one XML schema defining attributes of data to be queried; at least one generator programmed to generate query statements in XML Query Language; and a query assembler, wherein the query assembler invokes the generator to assemble and submit query requests to multiple different databases at least some of which are not homogeneous for the execution of the specified query upon a collection of data and wherein the collection of data is populated with instances of the at least one XML data schema.