A concept to introduce external models in object oriented databases is presented, such that application programs do no longer interface directly the whole conceptual schema, but work against external schemas specifically designed for the applications requirements. There are virtually no restrictions for such applications, since the interaction with the database takes place via updateable views. The data model is a somewhat simplified form of ODMG-93 [Catt93], where we incorporated the additional constructs we need for the external schema definition. The approach makes a clear distinction between types and classes, and also separates the type and class hierarchies of the conceptual schema from the external type and class hierarchies. With type derivation, we provide a powerful type restructuring mechanism, which allows to define an external type which is based on a conceptual type. In the derivation process, one can omit conceptual components and methods or redefine their types. Additional methods can be defined for external types as well. By defining well formed external schemas via constraints and schema invariants, we are able to guarantee unambiguous method resolution, steadiness of method resolution and compliance with the covariant subtyping principle. The full semantics of the conceptual schema are preserved. The designer of the external schema can make use of all information contained in the conceptual schema, in particular conceptual methods can be called from externally defined ones. In this paper, we concentrate on the area of type derivation and method resolution.

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