1. A system for data integration by querying multiple extensible markup language (XML) source schemas through a common XML target schema, said system comprising: XML mapping connections between said XML source schemas and said XML target schema, wherein said XML mapping connections accommodate XML mappings, said XML source schemas comprise data, and said XML target schema comprise a set of constraints; a target query associated with said XML target schema; and a query rewriter adapted to reformulate said target query in terms of said XML source schemas based on said XML mappings, and to integrate said data based on said set of constraints.