Co-Occurrence Constraints

Introduction

A co-occurrence constraint specifies the minimum and maximum number of times an object or combination of objects can co-occur in the relationships of a relationship set with another object or combination of objects.

Notationally, we write co-occurrence constraints by listing one or more object class names on the left and right of a right-pointing arrow and by providing a list of min:max pairs above the arrow. In an ORM, we place a co-occurrence constraint close to the relationship set to which it applies.

For instance:

Figure 1. Sample co-occurrence constraints

This example means that each triple object student, course and semester in respective object class Student, Course and Semester can have one and only one grade in object class Grade. If the minimum and maximum numbers above the arrow are all 1s, then this is the same as generalized Functional Dependency.

To properly form co-occurrence constraints, several restrictions must be observed:

• The object-class names in the left and right list of a co-occurrence constraint must be a subset of the object-class names in the relationship set.
• Neither the left list nor the right list may be empty.
• An object-class name may not be listed more than once.
• Minimums cannot be zero.