The relationship strength is important to determine how the primary key of a related identity is defined. There are instances when the primary key of one entity only appears as the foreign key in the related entity. In those cases, you have a weak relationship. There are other times when the foreign key is also the primary key component in the related entity, which creates a strong relationship. Chen’s notation does not distinguish in a diagram the difference between a weak and strong relationship. In Crow’s foot notation, a dashed relationship line is used to signify a weak relationship. This is not always used in diagraming ERDs. However, it is useful to know this if you do see it in a diagram.
You identified the strength of entities back in Unit 4, entities in ERDs can also be strong or weak. A strong entity can exist without any dependency on any other relationship. Typically, these entities do not have any foreign keys. These tables are ones that you would typically create first since they don’t depend on other entities. In fact, other entities typically depend on them.
Weak entities meet one of two criteria:
In Chen’s notation, a weak entity is signified by a double wall on the entity (a double rectangular box).
EXAMPLE
For example, look at the Rating entity in your current design (below). The Rating entity is existence-dependent. It has dependencies on both the user and the movie. A rating cannot exist without having a user that submitted the rating. A rating also cannot exist without being connected to a movie. You have not normalized the database yet to create a bridge entity between movie/actor and movie/genre, but those bridge entities would also be weak entities.
Source: Authored by Vincent Tran
