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 aggregation versus ternary relationship. In short, each Project entity is sponsored by one or more Department entities and each Department can sponsor zero, one or more Projects. Each Sponsorship relationship has a Monitors relationship, which connects Employees with Sponsorship.

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enter image description here 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 Sponsors2 to Emplyees in ternary relation. Does this not satisfy the additional constraint?


No it can't.

Problem is the relationship Monitors. Every relationship has to be between two entities.

Employee is not related to Projects nor Departments, but to the aggregate of the Projects and Departments which forms a new entity which is drafted with the dotted line

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