Strong Entity
Strong Entity is independent to any other entity in the schema. A strong entity always have a primary key. In ER diagram, a strong entity is represented by rectangle. Relationship between two strong entities is represented by a diamond. A set of strong entities is known as strong entity set.
Weak Entity
Weak entity is dependent on strong entity and cannot exists without a corresponding strong. It has a foreign key which relates it to a strong entity. A weak entity is represented by double rectangle. Relationship between a strong entity and a weak entity is represented by double diamond. The foreign key is also called a partial discriminator key.
Following are some of the important differences between Strong Entity and Weak Entity.
| Sr. No. | Key | Strong Entity | Weak Entity |
|---|---|---|---|
| 1 | Key | Strong entity always have one primary key. | Weak entity have a foreign key referencing primary key of strong entity. |
| 2 | Dependency | Strong entity is independent of other entities. | Weak entity is dependent on strong entity. |
| 3 | Represented by | A strong entity is represented by single rectangle. | A weak entity is represented by double rectangle. |
| 4 | Relationship Representation | Relationship between two strong entities is represented by single diamond. | Relationship between a strong and weak entity is represented by double diamond. |
| 5 | Participation | Strong entity may or may not participate in entity relationships. | Weak entity always participates in entity relationships. |