I am reading a book about database systems and in the chapter covering database design using ER, the author aims to explain when to use
ternary relationship. In short, each
Project entity is sponsored by one or more
Department entities and each
Department can sponsor zero, one or more
Sponsorship relationship has a
Monitors relationship, which connects Employees with Sponsorship.
This can be expressed in 2 ER diagrams:
Now, we want to express an additional constraint that each
Sponsorship relationship is monitored by at most one
Employee.The author states that this cannot be done with ternary relationship. I can't wrap my head around why. To express this in diagrams, I would just point an arrow from
Emplyees in ternary relation. Does this not satisfy the additional constraint?