Introduction

Relationships are even more fundamental than the above quotation indicates. They have been around since the beginning of time. Since there have been “things,” there have been relationships among them. And, since humans have been recording in-formation about things, we have been recording relationships. After all, an atom “contains” one or more subatomic particles—a relationship. A figure in a prehistoric cave painting “is adjacent to” some other figure, likely recording some meaningful relationship in the mind of the ancient artist.

A relationship is a named correspondence between objects. Much work in data modeling has focused on understanding the many types of relationships that naturally surface in any application. Relationships are one of the most fundamental parts of any data model. From one point of view, they are what distinguishes databases from file systems.

(Zdonik and Maier 1990, p. 21)