How to identify a strong vs weak relationship on ERD?

entity-relationship erd
CHEBURASHKA picture CHEBURASHKA · May 21, 2013 · Viewed 130.5k times · Source

A dashed line means that the relationship is strong, whereas a solid line means that the relationship is weak. On the following diagram how do we decide that the relationship between the Room and Class entities is strong. Is it because Room entity has a regular key (non-composite)? enter image description here

Answer

user2333862 picture user2333862 · May 21, 2013

  1. Weak (Non-Identifying) Relationship

    • Entity is existence-independent of other enties

    • PK of Child doesn’t contain PK component of Parent Entity

  2. Strong (Identifying) Relationship

    • Child entity is existence-dependent on parent

    • PK of Child Entity contains PK component of Parent Entity

    • Usually occurs utilizing a composite key for primary key, which means one of this composite key components must be the primary key of the parent entity.

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