Database Reference
In-Depth Information
Unary relationships have only one participant the relation is associated with itself. These
relationships can exist between entity instances of the same type. These are also called re-
cursive relationships. An example of a unary relationship is Employee to Manager. One's
manager is, in most cases, also an employee with a manager of his or her own. Unary rela-
tionships can be of any cardinality. One-to-many unary relationships are used to implement
hierarchies, such as the organizational hierarchy implicit in the Employee-Manager rela-
tionship.
Example 1:
In following figure the line drawn between 'Computer Engg.' and '230' indicates that lec-
turer 230 is employed by the Computer Engg department. Note that the cardinality of the
relationship is 1:M. The department 'Computer Engg', for instance, has two lecturers asso-
ciated with it. The entity Lecturer has mandatory participation while Department has op-
tional participation.
Electrical Engg., for instance, is not associated with any lecturers.
