How to identify a strong vs weak relationship on ERD?

Question

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.

We draw a solid line if and only if we have an ID-dependent relationship; otherwise it would be a dashed line.

Consider a weak but not ID-dependent relationship; We draw a dashed line because it is a weak relationship.

The relationship Room to Class is considered weak (non-identifying) because the primary key components CID and DATE of entity Class doesn't contain the primary key RID of entity Room (in this case primary key of Room entity is a single component, but even if it was a composite key, one component of it also fulfills the condition).

However, for instance, in the case of the relationship Class and Class_Ins we see that is a strong (identifying) relationship because the primary key components EmpID and CID and DATE of Class_Ins contains a component of the primary key Class (in this case it contains both components CID and DATE).

In an ER diagram, I believe when the relationship is strong, i.e., the primary_key of the parent forms a composite or non-composite primary_key in the child entities, we use a solid line to depict that. Similarly, for the case when the relationship is weak, which means the primary_key of the parent is not used as a primary_key in the child entity, then we use a dashed line to show that.