1. A system enabling the querying of data modeled by an XML schema, comprising: a processor; A non-transitory computer readable media coupled to the processor, the computer readable media comprising: at least one XML data schema describing contents of data to be queried, wherein the XML data schema is used to generate a query form user interface, wherein the query form user interface comprises an input portion and an output portion; at least one generator programmed to generate query statements in XML Query Language in response to user input in the query input user interface; and a query assembler, wherein the query assembler invokes the generator to assemble and submit query requests received at the input portion 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, and wherein a response to the query requests is displayed on the output portion; wherein the input portion of the query form comprises query form controls that are mapped to the XML data schema; wherein a first query form control comprises an instance of a control schema defined by an XML Schema Definition Language file and maps to a specific element of the XML data schema and identifies one or more subquery generators.