7.4.2: Participation

Suppose we are designing a database for a company that has several departments and employees. Suppose further that each employee must be assigned to work in one department. We can define a works in relationship involving Department and Employee. Employees must participate in the relationship, and we show this using a double line joining the diamond symbol to the Employee entity type.

        Figure 7.18: Employee must work in a department

The double line stands for total or mandatory participation which means that instances of the adjacent entity type must participate in the relationship – in the case above, all instances of Employee must be assigned to some department. Any time we show a single line we are stating participation is optional; for the above we are saying that a department will have zero or more employees who work there.


Share This Book