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 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.|