How to identify a strong vs weak relationship on ERD?
Solution 1
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.
Solution 2
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
).
Comments
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CHEBURASHKA over 2 years
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
andClass
entities is strong. Is it becauseRoom
entity has a regular key (non-composite)?