What is the Difference Between Weak Entity and Strong Entity

The main difference between weak entity and strong entity is that weak entity depends on a strong entity while a strong entity is an entity that does not depend on another entity.

ER diagrams help to visually represent databases. In ER modeling, we can create entities, attributes, and relationships in order to understand how data is related to each other. Moreover, entity is an important concept used in ER diagrams. It is a real-world object that represents data. More importantly, attributes are the characteristics of an entity. There are two types of entities called strong entity and weak entity. In brief, weak entity always depends on a strong entity.

Key Areas Covered

1. What is a Weak Entity
     – Definition, Functionality
2. What is a Strong Entity
    – Definition, Functionality
3. What is the Difference Between Weak Entity and Strong Entity
    – Comparison of Key Differences

Key Terms

Entity, ER Diagrams, Strong Entity, Weak Entity

What is a Weak Entity

A weak entity is an entity that depends on a strong entity. For example, in a university database, there are entities called course and exam. The exam is based on the course. Exam name, questions, etc. depend on the course. Therefore, the course entity is a strong entity because it does not rely on some other entity. On the other hand, exam is a weak entity as it relies on the strong entity.

A weak entity does not have a primary key. It has a composite key, which is a primary key of a strong entity and a partial key of the weak entity. A set of weak entities create a weak entity set. In an ER diagram, a double rectangle represents a weak entity. Furthermore, the relationship between a strong entity and a weak entity is a denoted by a double diamond.

What is a Strong Entity

A strong entity is an entity that does not depend on another entity. For instance, assume a university management system. It stores details about students and lecturers. There are separate tables to enter student records and lecturer records. Student and lecturer are entities in the ER diagram. Similarly, a medical center management system store details about patients, doctors,  etc. Patients and doctors are separate tables in the database. Thus, these are strong entities.

A strong entity has a primary key. Therefore, we can identify each entity in a strong entity set. A set of strong entities create a strong entity set. In ER diagrams, a rectangle is used to denote a strong entity. Furthermore, a single diamond represents the relationship between two strong entities.

Difference Between Weak Entity and Strong Entity

Definition

A weak entity is an entity that cannot be uniquely identified by its attributes alone whereas a strong entity is an entity that is independent of any other entity in a schema. Thus, this is the main difference between weak entity and strong entity.

Dependency

Importantly, a weak entity always depends on a strong entity, but a strong entity does not depend on another entity.

ER Diagram Representation

Another difference between weak entity and strong entity is that a double rectangle denotes a weak entity while a rectangle represents a strong entity.

Key Attributes

Moreover, a weak entity does not have any key attribute on its own while a strong entity has a key attribute. Hene, this is an important difference between weak entity and strong entity.

Conclusion

In brief, an entity is a real-world object that represents data. Moreover, there are two types of entities as weak entity and strong entity. The main difference between weak entity and strong entity is that weak entity depends on a strong entity while a strong entity is an entity that does not depend on another entity.

Reference:

1. “Working with ER Diagrams.” Studytonight, Available here.
2. “Weak Entity.” Wikipedia, Wikimedia Foundation, 3 Apr. 2019, Available here.

Image Courtesy:

1. “Weak entity ER-example” By Mixer – Using Dia software especially for Wikipedia’s “Weak entity” article (CC BY-SA 3.0) via Commons Wikimedia