Report Number: CS-TR-79-722
Institution: Stanford University, Department of Computer Science
Title: A structural model for database systems
Author: Wiederhold, Gio
Author: El-Masri, Ramez A.
Date: February 1979
Abstract: This report presents a model to be used for database design.
Because our motivation extends to providing guidance for the
structured implementation of a database, we call our model
the 'Structural Model.' We derive the design using criteria
of correctness, relevance, and performance from semantic and
operational specifications obtained from multiple sources.
These sources typically correspond to prospective users or
user groups of the database. The integration of such
specifications is a central issue in the development of an
integrated structural database model.
The structural model is used for the design of the logical
structures that represent a real-world situation. However, it
is not meant to represent all possible real-world semantics,
but a subset of the semantics which are important in database
The model uses relations as building blocks, and hence can be
considered as an extension of Codd's relational model .
The main extensions to the relational model are the explicit
representation of logical connections between relations, the
inclusion of insertion-deletion constraints in the model
itself, and the separation of relations into several
Connections between relations are used to represent existence
dependencies of tuples in different relations. These
existence dependencies are important for the definition of
semantics of relationships between classes of real-world
entities. The connections between relations are used to
specify these existence dependencies, and to ensure that they
remain valid when the database is updated. Hence, connections
implicitly define a basic, limited set of integrity
constraints on the database, those that identify and maintain
existence dependencies among tuples from different relations.
Consequently, the rules for the maintenance of the structural
integrity of the model under insertion and deletion of tuples
are easy to specify.
Structural relation types are used to specify how each
relation may be connected to other relations in the model.
Relations are classified into five types: primary relations,
referenced relations, nest relations, association relations,
and lexicon relations. The motivation behind the choice of
these relation types is discussed, as is their use in data
A methodology for combining multiple, overlapping data models
- also called user views in the literature - is associated
with the structural model. The database model, or conceptual
schema, which represents the integrated database, may thus be
derived from the individual data models of the users. We
believe that the structural model can be used to represent
the data relationships within the conceptual schema of the
ANSI/SPARC DBMS model since it can support database
submodels, also called external schema, and maintain the
integrity of the submodels with respect to the integrity
constraints expressable in the structural model.
We then briefly discuss the use of the structural model in
database design and implementation. The structural model
provides a tool to deal effectively with the complexityu of
large, real-world databases.
We begin this report with a very short review of existing
database models. In Chapter 2, we state the purpose of the
model, and in Chapter 3 we describe the structural model,
first informally and then using a formal framework based on
extensions of the relational model. Chapter 4 defines the
representations we use, and Chapter 5 covers the integration
of data models that represent the different user
specifications into an integrated database model. Formal
descriptions and examples of the prevalent cases are given.
The work is then placed into context first relative to other
work (Chapter 6) and then briefly within our methodology for
database design (Chapter 7).