Explain difference between Strong Entity and Weak Entity

In database design, entities are classified as strong or weak based on their independence. A strong entity can exist on its own with a primary key, while a weak entity depends on a strong entity and cannot be uniquely identified without it.

Strong Entity

A strong entity is independent of any other entity in the schema. It always has a primary key that uniquely identifies each instance. In an ER diagram, a strong entity is represented by a single rectangle, and a relationship between two strong entities is shown as a single diamond.

Weak Entity

A weak entity depends on a strong entity and cannot exist without it. It uses a partial key (discriminator) combined with the strong entity's primary key to form a composite key. In an ER diagram, a weak entity is represented by a double rectangle, and its relationship with the strong entity is shown as a double diamond.

ER Diagram Notation

Key Differences

Conclusion

A strong entity exists independently with its own primary key, while a weak entity depends on a strong entity and requires a composite key for unique identification. In ER diagrams, weak entities and their relationships are always shown with double borders to distinguish them from strong entities.