
Properties of relationships
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An important property of a relationship is its degree. Suppose there is a relationship
Teaches between the entities Lecturer and Course; a lecturer is identified by
lecturerName, and a course by courseNo. There are three possible kinds of relationship
degree, each corresponding to different pairs of enterprise rules for the relationship.
1:1 relationship
Enterprise rules:
‘A lecturer teaches, at most, one course.’
‘A course is taught by, at most, one lecturer.’
This pair of enterprise rules defines Teaches as being a 1:1 relationship. (Read 1:1 as
1-to-1.) A typical occurrence diagram is shown ...