Page 252 - DCAP405_SOFTWARE_ENGINEERING

Unit 14: Data Modeling

Notes

Notes A one-to-one relationship between two entity sets does not imply that for an
occurrence of an entity from one set at any time there must be an occurrence of an entity
in the other set. In the case of an organization, there could be times when a department is
without a manager or when an employee who is classified as a manager may be without
a department to manage.
Figure 14.8 shows some instances of one-to-one relationships between the entities DEPARTMENT
and MANAGER.

Figure 14.7: One-to-One Relationship

Figure 14.8: An instance of One-to-One Relationship

Following are the examples of the same relationship under different cardinality constraints
(figure 14.9).

Figure 14.9: (a), (b), (c), (d), (e) and (f)

(a) A Department is managed by no Manager

(b) One Department is managed by exactly one Manager

(c) One Department is managed by many Managers

(d) One Department is managed by one or more Managers
Contd...

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