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One-to-Many Unary Relationship in DBMS
A One-to-Many Unary Relationship is the association with the same entity between the same instances represented by different role groups.
Consider the relationship between Managers and Employees.
The two different Roles in this relationship are- Managers and Employees. Only few Employees take the role of Managers within an organization while everyone working for an organization is an Employee.
One Manager manages multiple Employees and each Manager is also an Employee.
One-to-Many Optional-Mandatory
Each instance of one role group must participate in the relationship while instance of other role group can optionally participate in the relationship.
The Role of Manager satisfies the optional participation as there are Employees as managers and non-managers. The Employee role group violates the role uniqueness because at least one employee at the top level of management doesn’t have a manager.
One-to-Many Optional-Optional
The One-to-Many Optional-Optional is valid in this case as Manager Role is optional for the Employees and a senior top most management Employee is not managed by other instances in the entity.
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