Database abstractions: aggregation and generalization

ABSTRACT

Two kinds of abstraction that are fundamentally important in database design and usage are defined. Aggregation is an abstraction which turns a relationship between objects into an aggregate object. Generalization is an abstraction which turns a class of objects into a generic object. It is suggested that all objects (individual, aggregate, generic) should be given uniform treatment in models of the real world. A new data type, called generic, is developed as a primitive for defining such models. Models defined with this primitive are structured as a set of aggregation hierarchies intersecting with a set of generalization hierarchies. Abstract objects occur at the points of intersection. This high level structure provides a discipline for the organization of relational databases. In particular this discipline allows: (i) an important class of views to be integrated and maintained; (ii) stability of data and programs under certain evolutionary changes; (iii) easier understanding of complex models and more natural query formulation; (iv) a more systematic approach to database design; (v) more optimization to be performed at lower implementation levels. The generic type is formalized by a set of invariant properties. These properties should be satisfied by all relations in a database if abstractions are to be preserved. A triggering mechanism for automatically maintaining these invariants during update operations is proposed. A simple mapping of aggregation/generalization hierarchies onto owner-coupled set structures is given.

REFERENCES

• 1: CODD, E.F. Further normalization of the data base relational model. In Courant Computer Science Symposium 6: Data Base Systems, Prentice-Hall, Englewood Cliffs, N.J., May 1971, pp. 33-64.
• 2: HOAttE, C.A.R. Notes on data structuring. In A PIC Studies in Data Processing No. 8: Structured Programming, Academic Press, New York, 1972, pp. 83-174.
• 3: QUILbI~N, M.R. Semantic memory. In Semantic Information Processing, M.I.T. Press, Cambridge, Mass., 1968, pp. 227-268.
• 4: John Miles Smith , Diane C. P. Smith, Database abstractions: aggregation and generalization, ACM Transactions on Database Systems (TODS), v.2 n.2, p.105-133, June 1977 [doi>10.1145/320544.320546]

INDEX TERMS

Primary Classification:
  D. Software
  D.2 SOFTWARE ENGINEERING
        D.2.11 Software Architectures
            Subjects: Data abstraction

Additional Classification:
  H. Information Systems
  H.2 DATABASE MANAGEMENT
      H.2.1 Logical Design
          Subjects: Data models
      H.2.4 Systems
          Subjects: Relational databases

General Terms:
Design

Keywords:
aggregation, data abstraction, data model, data type, database design, generalization, integrity constraints, knowledge representation, relational database