# Difference between Strong Entity and Weak Entity

An entity is similar to a unique object and a collection of attributes. There are two types entities namely strong entity and weak entity. A strong entity is an independent entity. On the other hand, a weak entity is a dependent entity that depends on a strong entity.

## What is a Strong Entity?

A strong entity is complete by itself and is not dependent on any other entity type. It possess a primary key which describes each instance in the strong entity set uniquely. That means any element in the strong entity set can be uniquely identified. A strong entity is represented by a square with a single line unlike a Weak Entity which contains double lines.

## What is a Weak Entity?

A weak entity cannot be used independently as it is dependent on a strong entity type known as its owner entity. Also, the relationship that connects the weak entity to its owner identity is called the identifying relationship.

A weak entity always has a total participation constraint with respect to its identifying relationship because it cannot be identified independently of its owner identity. A weak entity may have a partial key, which is a list of attributes that identify weak entities related to the same owner entity.

In the ER diagram, both the weak entity and its corresponding relationship are represented using a double line and the partial key is underlined with a dotted line. In the given ER diagram, Dependent is the weak entity and it depends on the strong entity Employee via the relationship Depends on.

There can be an employee without a dependent in the Company but there will be no record of the Dependent in the company systems unless the dependent is associated with an Employee.

## Difference between Strong Entity and Weak Entity

The following table highlights the important differences between strong entity and weak entity −

Factor Strong Entity Weak Entity
Definition A strong entity is complete by itself and is not dependent on any other entity type. A weak entity cannot be used independently as it is dependent on a strong entity type known as its owner entity.
Nature Strong entity is independent in nature. Weak entity is dependent in nature. That means, it depends on the strong entity.
Primary key Strong entity has primary key. Weak entity does not have any primary key. But, it has a partial discriminator key.
Key attribute Strong entity has a key attribute. Weak entity does not have any key attribute.
Representation Strong entity can be represented by using a single rectangular box. Weak entity can be represented using a double rectangular box.
Participation Strong entity can either have no participation or total participation. Weak entity always have total participation.
Relationship between two entities The relationship between two strong entities is represented by a single diamond. The relationship between a weak entity and a strong entity is represented by a double diamond.

## Conclusion

The most significant difference between these two types of entities is that a strong entity has a primary key and does not depend on any other entity, whereas a weak entity has a partial discriminator key and it depends on another strong key.