The answers then will be translated back into the correct form of the answer of the original query. The external systems are assumed to be autonomous, with their own data models, query languages, and schema designs. The schema integration problem can be decomposed into two subproblems, schema restructuring and data integration. Schema restructuring generally is needed to remove semantic incompatibilities in the schemas to be integrated. Semantic incompatibilities are removed by transforming one or more schemas to be integrated into equivalent schemas where the incompatibilities are removed.
Equivalence is a mathematically precise notion.
Informally, two schemas are equivalent if either one can simulate the other in terms of capacities to store database instances and support queries. Although schema equivalence is a fundamental concept concerning databases, very little is known about it. One goal of this research effort is to develop further our knowledge of schema equivalence to serve as a theoretical foundation of schema integration. A major part of this goal is to develop syntactic characterizations of schema equivalence in the presence of various families of dependencies. The view is not populated with data, nor is a global database materialized.