The relationship strength is important to determine how the primary key of a related identity is defined. There are instances where the primary key of one entity only appears as the foreign key in the related entity, in those cases, we have a weak relationship. There are other times where the foreign key is also the primary key component in the related entity which creates a strong relationship. With Chen’s notation, it does not distinguish in a diagram of 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.
The other aspect to consider in ERDs is strong and weak entities. A strong entity can exist without any dependency on any other relationship. Typically, these entities are ones that do not have any foreign keys. These tables are the ones that you would typically create first since they don’t depend on other entities. Other entities will typically depend on them. Weak entities are ones that meet one of two criteria. The first criterion that would define a weak entity is if it is existence-dependent. This means that it cannot exists without the entity that it has a relationship with. The other criteria of a weak entity are if the entity has a primary key that is partially or entirely derived from the parent entity in the relationship. This means that if there is a strong relationship, the entity that derives from the other table is a weak entity.
In Crow’s foot notation, the relationship line along with the primary key/foreign key is used to signify the weak entity. In Chen’s notation, the weak entity is defined by a double wall on the entity. This would be the case for the Rating entity in our current design. We have not normalized the database yet to create the bridge entity between movie/actor and movie/genre but those bridge entities would also be weak entities. The Rating entity will have dependencies on both the user and the movie. A rating cannot exist without having the user that submitted the rating. A rating cannot exist without being connected to a movie either.