ERD, Entity, Strong Entity VS Weak Entity

• ERD

An Entity Relationship Diagram

A type of flowchart that illustrates how “entities” such as people, objects or concepts relate to each other within a system.

For instance, if there are writer, novel, consumer, they can be dipicted as below.

 

• Entity

A definable thing—such as a person, object, concept or event—that can have data stored about it. Think of entities as nouns. Examples: a customer, order, product.

Typically shown as a rectangle.

 

• Difference between Strong and Weak Entity

- Strong Entity

A strong entity is not dependent on any other entity. A strong entity will always have a primary key. Strong entities are represented by a single rectangle. The relationship of two strong entities is represented by a single diamond. 
Various strong entities, when combined together, create a strong entity set. 

 

- Weak Entity

A weak entity is dependent on a strong entity to ensure its existence. Unlike a strong entity, a weak entity does not have any primary key. It instead has a partial discriminator key. A weak entity is represented by a double rectangle. 
The relation between one strong and one weak entity is represented by a double diamond. 

Strong Entity	Weak Entity
Strong entity always has a primary key.	While a weak entity has a partial discriminator key.
Strong entity is not dependent on any other entity.	Weak entity depends on strong entity.
Strong entity is represented by a single rectangle.	Weak entity is represented by a double rectangle.
Two strong entity’s relationship is represented by a single diamond.	While the relation between one strong and one weak entity is represented by a double diamond.
Strong entities have either total participation or not.	While weak entity always has total participation.