A weak entity is an entity whose occurrences are dependent for their existence, through a relationship
R, on the occurrence of another (strong) entity.
The primary identifier
lineno for the weak entity line_item is only meaningful within some order.
This implies the primary identifier for the line_item must include the attributes in the primary identifier for the order entity.
†Multi-valued attribute (
hobbies) could be identified as a weak entity, but not vice verse.