7 The Weak Entity
7.1 INTRODUCTION
Chapters 4 and 5 introduced the concepts of the entity, the attribute, and the relationship. Chapter 6 dealt with structural constraints, that is, how two entities are related to one another. In this chapter, we discuss the concept of the “weak” entity, which is used in the Chen-like model. Weak entities may not have a key attribute of their own as they are dependent on a strong or regular entity for their existence. Strong entities always have a primary key. The weak entity has some restrictions on its use and generates some interesting diagrams. This chapter revisits and redefines steps 3 and 4 of the entity-relationship (ER) design methodology to include the concept ...
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