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 Project
s. Each 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 Sponsors2
to Emplyees
in ternary relation. Does this not satisfy the additional constraint?