Classification and transformations of binary relationship relation schemata

摘要

In contrast to Relational Model that deals only with flat relations, models like Entity-Relationship Model, Network Model and Hierarchical Model make distinctions between entity relations and relationship relations. Distinctions between these two types of relations are usually perceived intuitively. However, to establish a formal basis of data translations among different data models, it seems necessary to describe such distinctions in a precise manner. In this paper, relationship relations are defined as those on which relationship rules hold. Binary relationship relations are of special importance because these can be regarded as correspondences and also as directed graphs. Cardinality of correspondences and acyclicness and connectedness of directed graphs are important properties of binary relationship relations. With these properties taken into consideration a classification of binary relationship relation schemata can be established. Finally, four lossless transformations between two groups of binary relationship relation schemata are described. These play important roles in data translations among different data models and also in devising physical representation of binary relationship relations.