US Patent 7512642 Mapping-based query generation with duplicate elimination and minimal union

A method and system for generating a query implementing a schema mapping. A mapping M is provided from a schema S to a schema T, where M relates S to T, and M includes a plurality of constraints. Schemas S and T each include one or more elements, and T includes at least one set type element. Mapping M is expressed in terms of at least one nested tuple-generating dependency. A query Q is generated where Q is capable of applying M to an input instance I to result in an output instance J, where I conforms to S, J conforms to T, and I and J satisfy the plurality of constraints. Instance J is in partitioned normal form (i.e., satisfies minimal union semantics) and includes no duplicate element instances.