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Acta Informatica 19, 4 (Sept. 1983)  Acta Informatica 44:1983.Type:Journal
Date Reviewed: Mar 1 1986

The Entity-Relationship (ER) model introduced by Chen in 1976 [1] provided a marked improvement in our ability to express the semantics of data, but left a number of modeling problems unsolved. Investigators since then have been enhancing the ER model in an attempt to solve these problems. This paper describes such an enhancement.

The modeling problems addressed by this paper are: (1) the modeling of subclasses or “IS-A” hierarchies; (2) the modeling of superclasses or generalization hierarchies; and (3) the use of entities of different types in the same position of a relationship. To solve these problems, the authors introduce a single new construct, the category. A category is a generalization of an entity type: it may be a subset of an entity type (e.g., SCIENTIST is a subset of EMPLOYEE), thus solving the subclass modeling problem; or it may be a union of several entity types (e.g., OWNER is a union of PERSON and CORPORATION), thus solving the superclass problem. A category may participate in relationships. Since a category may contain entities of different types, the problem of different entity types in the same position of a relationship is also solved.

In the ECR model, only categories may participate in relationships. If there is a need for an entity type in a relationship, it must first be defined as a (degenerate) category. This is a key simplifying feature of the method.

The paper covers a lot of ground in a relatively short space. The Introduction gives a good account of DBMS models, their problems, and the evolution of semantic models to overcome these problems. The ECR model is defined, and the representation of ECR schemas in both the relational model and the DBTG model is described. Two extensive examples of ECR modeling are given: an airport database and a company database. Finally, a language (GORDAS) for querying and updating ECR databases is described.

One of the criticisms often leveled at ER modeling is the difficulty of expressing query operations; in particular, the ability to express queries which span more than a single entity without nesting of selection expressions. GORDAS attacks this problem by permitting relationships to be labeled with “connection names,” and permitting these names to be used in queries in much the same way that attribute names would be used: the first level relationships of an entity become, in effect, attributes of that entity. Nesting of selection expressions is still required if relationships are followed, but the frequency of nesting is greatly reduced.

The diagramming techniques proposed for ER schemas have a common problem: they are impossible to reproduce faithfully on the average printer or display panel. ECR is no exception, using such figures as circles and set-inclusion symbols. A special problem arises in ECR when a diagram shows an entity type which is both a category and the base for other categories: should it be drawn as a category (a hexagon) or as an entity type (a rectangle)?

The paper is generally well written. A confusing statement appears early in the paper that categories are defined with respect to the roles they play in relationships; it appears, however, that categories can be defined and understood independently of any relationships. The examples contain only binary relationships. Had the authors included relationships of order greater than two, they could have shed some light on the question of whether such relationships complicate the manipulation of ER data.

Reviewer:  W. C. McGee Review #: CR109847
1) Chen, P. P.The entity-relationship model: towards a unified view of data, ACM Trans. Database Syst. 1 (1976), 9–36.
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