A system and method for data integration by querying multiple extensible markup language (XML) source schemas through a common XML target schema, wherein the system comprises XML mapping connections between the XML source schemas and the XML target schema, wherein the XML mapping connections accommodate XML mappings, the XML source schemas comprise data, and the XML target schema comprise a set of constraints, which comprise data merging rules for integrating the data from multiple source schemas comprising overlapping information; a target query associated with the target schema; and a query rewriter adapted to reformulate the target query in terms of the source schemas based on the mappings, and to integrate the data based on the set of constraints. The query rewriter is adapted to rewrite the target query into a set of source queries comprising the source schemas. A processor evaluates a union of the set of source queries.