Overview
An entity set is a group of entity instances of a particular entity type. For example, all employees of an organization form an entity set.
Types of Entity Sets
The different types of entity sets are as follows:
Entity types(ETs) can be classified into the following types:
- Strong Entity
- Weak Entity
Strong Entity
A strong entity type can exist independently and have its own identification, that is, without being linked to the instances of any other entity type. For example, an employee in the previous example is an independent or strong entity type since its instances can exist independently. Strong ETs are also called regular or independent ETs.
Strong entities are self-sufficient in that their existence does not depend on the presence of any other entities.
Weak Entity Type
An entity type whose instances cannot exist without being linked with instances of some other entity type, i.e., they cannot exist independently. Weak ETs are also called the dependent ETs.
For example, an organization wants to maintain data about the employees and their dependents. In this example, each Dependent is associated with a specific Employee and cannot exist without that Employee.
An identifying relationship exists between a strong and weak entity when the weak entity’s existence is contingent upon the strong entity. Suppose an employee leaves the job, and the organization deletes the employee’s record. In that case, the employee’s dependent records will also be deleted since they cannot exist without being linked to an employee.