Weak Entities (cont

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Weak Entities (cont)

• Entities that do not have key attributes of their
  own
• Related to specific entities from another entity
  type plus their own attributes
• Another Entity Type Owner Entity
• Weak Entity always has total participation
  (mandatory) with respect to Owner Entity
• Weak Entity cannot be identified without owner
  entity
• Partial Keys represent uniqueness within a
  certain owner entity
• One to Many Relationship one owner to many weak

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Generalization Hierarchy

• A generalization hierarchy is a structured
  grouping of entities that share common
  attributes. It is a powerful and widely used
  method for representing common characteristics
  among entities while preserving their
  differences. It is the relationship between an
  entity and one or more refined versions. The
  entity being refined is called the supertype and
  each refined version is called the subtype.
• Generalization hierarchies should be used when
  (1) a large number of entities appear to be of
  the same type (2) attributes are repeated for
  multiple entities (3) the model is continually
  evolving
• Generalization hierarchies improve the stability
  of the model by allowing changes to be made only
  to those entities germane to the change and
  simplify the model by reducing the number of
  entities in the model.

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Generalization Hierarchy
A generalization hierarchy can either be
overlapping or disjoint. In an overlapping
hierarchy an entity instance can be part of
multiple subtypes. For example, to represent
people at a university you have identified the
supertype entity PERSON which has three subtypes,
FACULTY, STAFF, and STUDENT. It is quite possible
for an individual to be in more than one subtype,
a staff member who is also registered as a
student. In a disjoint hierarchy, an entity
instance can be in only one subtype. For example,
the entity EMPLOYEE, may have two subtypes,
CLASSIFIED and WAGES. An employee may be one type
or the other but not both.
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Generalization Hierarchy