Optionality
A relationship can be optional or mandatory.
- If the relationship is mandatory
- an entity at one end of the relationship must be related to an entity at the other end.
- The optionality can be different at each end of the relationship
- For example, a student must be on a course. This is mandatory. To the relationship `student studies course' is mandatory.
- But a course can exist before any students have enrolled. Thus the relationship `course is_studied_by student' is optional.
- To show optionality, put a circle or `0' at the `optional end' of the relationship.
- As the optional relationship is `course is_studied_by student', and the optional part of this is the student, then the `O' goes at the student end of the relationship connection.