1. A system for automated data integration by querying multiple extensible markup language (XML) source schemas through a common XML target schema, said system comprising: a processor that: establishes XML mappings between said XML source schemas and said XML target schema, wherein said XML source schemas comprise data, and wherein said XML target schema comprise a set of constraints, said set of constraints comprising: data merging rules for integrating said data from multiple said XML source schemas comprising overlapping information; and nested equality-generating dependencies (NEGDs), said NEGDs comprising rules specifying that fields in two different data tuples must be merged; queries multiple XML source schemas through a common XML target schema; rewrites said target query in terms of said XML source schemas based on said XML mappings; integrates said data based on said set of constraints; and evaluates a union of a set of source queries, wherein an evaluation of said union of said set of source queries in said processor occurs at query run-time; and a display device adapted to display said target query and said data to a user.