7. ENTITY RELATIONSHIP MODELLING
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.
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.