Representing Weak Entities | Database Management System

Representing Weak Entities | Database Management System

Representing Weak Entities : An entity set that does not possess sufficient attributes to form a primary key is called a weak entity set.

A weak entity set does not have a primary key, but we need a means of distinguishing among the entities.

The discriminator of a weak entity set is a set of attributes that allows this distinction to be made.

  • A weak entity set cannot exist by itself.
  • It depends on an identifying entity set (strong entity set) via an identifying relationship Set.
  • An identifying relationship is depicted using a double diamond.
  • A weak entity set is depicted using a double rectangle.

For example in our banking example, there is a payment entity which cannot exist without loan entity and it does not have sufficient attributes to be qualified as primary key. Therefore, it is represented as weak entity as shown in the following figure.

 

The following E-R diagram represents a more complex model that includes a weak entity.

 

Fig. An E-R Diagram Showing a Weak Entity