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Information Modeling: The EXPRESS Way$
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Douglas Schenck and Peter Wilson

Print publication date: 1994

Print ISBN-13: 9780195087147

Published to Oxford Scholarship Online: November 2020

DOI: 10.1093/oso/9780195087147.001.0001

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Integration and specialization

Integration and specialization

Chapter 6 Integration and specialization
Information Modeling: The EXPRESS Way

Douglas Schenck

Peter Wilson

Oxford University Press

The major topic of this Chapter is modeling in the large. By this we mean looking at methods to integrate several different models into a single cohesive whole, and also examining the inverse problem of extracting a small specialized model from a larger one. Before we can sensibly discuss these, though, we need to look more closely at two aspects of EXPRESS, namely subtyping and schema interfacing. EXPRESS has a concept of Supertype and Subtype which taken together enable a type lattice to be constructed. A Subtype-Supertype relationship is typically called an ‘Isa’ relationship in data modeling terms. That is, a Subtype is a kind of its Supertype(s). For example, if we define an entity pet and also define two subtypes of this called cat and goldfish then an instance of cat is also an instance of pet, and similarly for a goldfish. The relationship, though, is asymmetrical, as a pet may be something other than a cat or goldfish; for instance a pet may also be a dog. A Subtype is a more specialized kind of thing than its Supertype and, conversely, a Supertype is a generalization of its Subtypes. A Subtype inherits all the attributes and constraints of its Supertype(s). In EXPRESS an entity is a Subtype of another entity if it declares its Supertype entity within its Subtype declaration. A Supertype does not declare its Subtypes. In general, an instance of a Subtype requires an instantiation of each of its Supertypes, while an instance of a Supertype does not require instantiation of its Subtypes. This later behavior may be modified by declaring the Supertype to be an Abstract Supertype, in which case an instance of the Supertype does require instantiation of at least one of its Subtypes. In most data modeling and Object Oriented languages that support similar notions to Subtyping, if an instance of a Supertype requires instantiation of a Subtype, then one and only one Subtype can be instantiated. EXPRESS does not have this restriction. Unless otherwise constrained, an instance of a Supertype may be accompanied by one instance of each of its Subtypes.

Keywords:   Abstract, Documentation, First-class, Generalization, Homonym, Import, OneOf, Partitioning, Quality, Reference

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