Subtype Discriminator

• Consider the problem of inserting a new instance of a supertype.

  • Into which of the subtypes (if any) should this instance be inserted?
  • We need a simple mechanism to implement these rules.
  • This mechanism is called Subtype Discriminator.

• A subtype discriminator : is an attribute of a supertype whose values determine the target subtype or subtypes.

  • Disjoint : A simple attribute with alternative values indicates the possible subtypes.
  • Overlapping : a composite attribute whose subparts relate to differe subtypes.
    • Each subpart contains a value indicting whether or not the instance belongs subtype.

Subtype Discriminator : Disjoint Subtypes

• Constraints have been added :

  • Total specialization and disjoint subtypes.

• Each employee must be either :

  • hourly,
  • salaried,
  • or a consultant.

• (Employee Type) attribute has been added to the supertype

  • As a subtype discriminator.

• When a new employee is added to the supertype, This attribute is coded with :

  • This attribute is coded with :
  • "H" (for Hourly)
  • "S" (for Salaried)
  • "C" (for Consultant).

• Depending on this code.

  • The instance is then assigned to the appropriate subtype.

Subtype Discriminator : Overlapping Subtypes

• When subtypes overlap, a slightly modified approach must be applied for the subtype discriminator.

• A given instance of the supertype may require that we create an instance in more than one subtype.

  • Example : PART and its overlapping subtypes.
  • A new attribute named Part Type has been added to PART. Part Type is a composite attribute with components Manufactured? and Purchased?
  • Each of these attributes is a Boolean variable

• When a new instance is added to PART, these components are coded as follows :

The values of the discriminator attribute parts will determine where the instance is going to be stored.