The Weak Entity
Chapters 2 and 3 introduced the concepts of the entity, the attribute, and the relationship. Chapter 4 dealt with structural constraints, that is, how two entities are related to one another. This chapter discusses 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 (that has a key attribute of its own). The weak entity has some restrictions on its use, and produces some interesting diagrams. This chapter revisits and redeﬁnes steps 3 and 4 of the ER design methodology to include the concept of the weak entity. A grammar for the weak entities and mapping rules for the weak entities are also developed.