Page 253 - DCAP405_SOFTWARE_ENGINEERING

Software Engineering

Notes

(e) A Department has none or exactly one Manager

(f) A Department has none or many Managers

A one-to-many relationship exists from the entity MANAGER to the entity EMPLOYEE because
there are several employees reporting to the manager. As we just pointed out, there could be an
occurrence of the entity type MANAGER having zero occurrences of the entity type EMPLOYEE
reporting to him or her. A reverse relationship, from EMPLOYEE to MANAGER, would be
many to one, since many employees may be supervised by a single manager. However, given
an instance of the entity set EMPLOYEE, there could be only one instance of the entity set
MANAGER to whom that employee reports (assuming that no employee reports to more than
one manager). The relationship between entities is illustrated in Figure 14.10.
Figure 14.10 (a) and (b)

(a) (M:1) Many Employees Report To one Manager

(b) An instance of (M:1) Relation

The relationship between the entity EMPLOYEE and the entity PROJECT can be derived as
follows: Each employee could be involved in a number of different projects, and a number of
employees could be working on a given project. This relationship between EMPLOYEE and
PROJECT is many-to-many. It is illustrated in Figure 14.11.

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