A One-to-One Unary Relationship is the association with the same entity between the same instances represented by same role group.
Above figure represents a set of married persons with the relationship MARRIED_TO. Each person is married to only one and only one person in the group.
In One-to-One Unary we have three cases available −
- Mandatory-Mandatory
- Optional-Optional
- Optional-Mandatory or Mandatory-Optional
Mandatory-Mandatory
Each instance of the role group must fully participate in the relationship. In the above example a Person is married to one and only one Person.
Optional-Optional
In this case the participation of a instance within a role group is optional i.e. the participation is not mandatory.
In the group married persons have the option to being taxed either individually or jointly. In this scenario at least one couple must pay their taxes separately.
Optional-Mandatory or Mandatory-Optional
Each instance of one role group must participate in the relationship while instance of other role group can optionally participate in the relationship.