Unary Relationships

---

Unary relationships associate occurrences of an entity type with other occurrences of the same entity type.

• e.g.,
  • entity Person
  • One person may be married to another person

Unary Relationships

One-to-One Unary Relationship

• Figure (a) shows a one-to-one unary relationship called Back-Up
  • involves Salesperson entity
  • salespersons are organized in pairs as backup to each other when one is away from work
  • in each direction the modality of one forbids the situation of a salesperson not having a backup

One-to-Many Unary Relationship

• a sales manager can manage several other salespersons
• each salesperson is managed by exactly one sales manager
• This situation describes a one-to-many unary relationship
• Figure (b)
  • The downward branch
    • It says that a salesperson manages zero to many other salespersons
      • means that
        • a salesperson may not be a sales manager (the zero modality case)
        • or may be a sales manager with several subordinate salespersons (the many cardinality case)
  • the rightward branch
    • says that a salesperson is managed by exactly one other salesperson (who must, of course, be a sales manager)

Many-to-Many Unary Relationship

Example

• One classic example of a many-to-many unary relationship is known as the “bill of materials” problem
  • Consider a complex mechanical object
  • Any such object is made of basic parts like nuts and bolts that are used to make other components or sub-assemblies of the object
  • Small sub-assemblies and basic parts go together to make bigger sub-assemblies, and so on until ultimately they form the entire object
  • Each basic part and each sub-assembly can be thought of as a “part” of the object
  • Then, the parts are in a many-to-many unary relationship to each other
  • Any one particular part can be made up of several other parts while at the same time itself being a component of several other parts

• Figure (c)
  • a product can be part of no other products or part of several other products
  • Going in the reverse direction, a product can be composed of no other products or be composed of several other products