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Difference between strong entity and weak entity

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Prerita Agarwal
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23 Jul, 2024 @ 01:30 PM

Introduction

Database design is an essential aspect of creating a well-structured and efficient database management system. One of the key concepts in database design is the distinction between strong entities and weak entities. 

difference between strong entity and weak entity

In this article, we will study the difference between strong entity and weak entity and their features. We will also see a difference table to understand the difference between them more clearly.

Also read about -  Entity and Attributes

What is Strong Entity?

A strong entity exists independently of any other entity in a data model. In other words, it has its unique identifier and does not depend on any other entity to be identified.

For example, consider a database for a university. The student entity is strong because it can exist independently of any other entity. It has its attributes, such as student ID, name, address, etc. On the other hand, the enrollment entity is not a strong entity because it depends on both the Student and Course entities to be identified. It has attributes such as enrollment, student, and course ID.

Strong entities have a primary key that uniquely identifies them and can interact with other entities in the data model.

Refer this to know about Yarn vs NPM here.

Features of Strong Entity

Some critical features of a strong entity are

  1. Unique identifier: A strong entity has its unique identifier that distinguishes it from other entities in the database. This identifier can be a single attribute or a combination of attributes uniquely identifying the entity.
     
  2. Participate in relationships: Since strong entities are independent, they can interact with other entities in the database. For example, in a university database, the Student entity can participate in a relationship with the Course entity to represent that a student can enroll in many courses.
     
  3. They can have attributes: They can have their attributes that describe their properties or characteristics. For example, the Student entity may have attributes such as name, address, phone number, and date of birth.
     
  4. Constraints: Constraints are rules that restrict the values that can be stored in a database. A strong entity can have constraints on its attributes, such as a constraint that ensures each student's ID attribute is unique.
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What is a Weak Entity?

A weak entity in database design do not have a unique identifier on its own and relies on a relationship with another entity to provide a primary key. A double rectangle denotes it and depends on a strong entity, also known as a parent entity, to create its primary key. Weak entities are important in database design because they enable the representation of complex relationships between entities, ensure data integrity, and avoid duplication of information.

Features of Weak Entity

Some critical features of a strong entity are

  1. Non-identifying: A weak entity does not have a primary key or unique identifier of its own.
     
  2. Dependent: It is dependent on a strong entity for its existence. This means that a weak entity cannot exist without the owner entity.
     
  3. Participation: A weak entity participates fully in a relationship with a strong entity. The relationship between the two entities is often one-to-many, where one owner entity can have multiple weak entities.
     
  4. Relationship: The relationship between a weak entity and its owner entity is represented by a solid diamond on the weak entity side and a double line on the owner entity side in an Entity Relationship Diagram (ERD).
     
  5. Attributes: A weak entity has attributes that are partially or wholly derived from the attributes of the strong entity. These attributes are used to create a composite key for the weak entity.
     
  6. Existence Dependency: A weak entity exists only when it is related to its owner entity. If the owner entity is deleted, all its dependent weak entities are also deleted.

Also see, Recursive Relationship in DBMS

Difference between strong entity and weak entity

Below table shows the difference between strong entity and weak entity.

 

Features

Strong Entity

Weak Entity

Unique Identifier

It has its own unique identifier known as primary key Do not have a unique identifier

Independence

It is independent It is dependent

Attributes

It has its own attributes that describe its characteristics. The attributes are described from attributes of strong entity

Relationship

It can have relationship with other entities It can only have a relationship with a strong entity

Participation

It can participate in any role Weak entity can only participate as a dependent entity

Existence Dependency

It can exist independently It depends on the existence of its owner entity

Representation

Independent entity with a primary key attribute. Dependent entity without a primary key attribute.

Relationship representation

Can have a direct relationship with other entities. Typically relies on a strong entity for existence.

Example

Employee, Customer Address, Order Item
A visual representation that the Ninja is a strong entity and programming languages are a weak entity.

In this entity relation diagram, a Ninja is a strong entity whereas the programming languages available on the platforms are a weak entity. 

Also see, Checkpoint in DBMS, Aggregation in DBMS

Frequently Asked Questions

Can a weak entity be related to two strong entities?

Yes, a weak entity can be related to two strong entities through two separate one-to-many relationships, linking it to both parent entities.

What is the relationship between two weak entities?

Two weak entities can be related through a one-to-one or one-to-many relationship, typically by sharing a common owner or parent entity, forming a hierarchical structure.

How are strong entities and weak entities related?

Strong entities and weak entities are connected by a one-to-many relationship. The strong entity is the "parent," and the weak entity is the "child." This is depicted in an ERD with a solid diamond on the weak entity and a double line on the strong entity.

Is a weak entity dependent on a strong entity?

Yes, a weak entity is dependent on a strong entity. It lacks a primary key of its own and relies on a strong entity for existence and identification.

Conclusion

In conclusion, the distinction between strong and weak entities is fundamental in database design. A strong entity can exist independently and has its unique identifier. In contrast, a weak entity depends on a strong entity for its existence and does not have a unique identifier. 

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Topics covered
1.
Introduction
2.
What is Strong Entity?
2.1.
Features of Strong Entity
3.
What is a Weak Entity?
3.1.
Features of Weak Entity
4.
Difference between strong entity and weak entity
5.
Frequently Asked Questions
5.1.
Can a weak entity be related to two strong entities?
5.2.
What is the relationship between two weak entities?
5.3.
How are strong entities and weak entities related?
5.4.
Is a weak entity dependent on a strong entity?
6.
Conclusion