Lesson 6: EER

• Supertype


  general entity type that has a relationship with 1 or more subtype
• Subtype


  specialized entity types to which the supertype is subdivided; subgrouping of the entities in an entity type.
• supertype and subtype ex

  EMPLOYEE becomes the supertype of SALARIED and HOURLY subtype.
• Attribute Inheritance


  Property by which subtype entities inherit values of all attributes and instance of all relationships of the supertype.
• There are attributes that apply to some (but not all) instances of an entity type.
• The instances of a subtype participate in a relationship unique to that subtype (a subtype has an attribute unique to a subtype)
• 2 processes used in developing supertype / subtype relationships

  1. Generalization
  2. Specialization
• Generalization


  defines more general entity type from a set of more specialized entity types; bottom up process
• Specialization


  Direct reverse of generalization; top down process