1. 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

2. 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.

3. 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.

4. Generalization Hierarchy