The Enhance E-R Model Essay Example

See if they can diagram these rules using the notation provided in this chapter. Answers to Review Questions

Define each of the following terms: . Supersets.

• generic entity type that has a relationship with one or more subtypes.
• Subtype. A suborning of the entities in an entity type that is meaningful to the organization.
• Specialization. The process of defining one or more subtypes of the supersets, and forming supersets/subtype relationships.
• Entity cluster. A set of one or more entity types and associated relationships grouped into a single abstract entity type.
• Structural assertion. A statement that expresses some aspect of the static structure of the organization.
• Anchor object. A business rule (a fact) on which actions are limited.
• Subtype coordinator. An attribute of the supersets whose values determine the target supersets or subtypes. . Total specialization rule. Specifies that each entity instance of the supersets must be a member of some subtype in the relationship.
• Generalization. The process of defining a generalized entity type from a set of more specialized entity types
• Disjoint rule. Specifies that if an entity instance (of the supersets) is a member of one subtype, it cannot simultaneously be a member of two (or more) subtypes.
• Overlap rule. Specifies that an entity instance can simultaneously be a member of two (or more) subtypes. . Action assertion.

A statement of a constraint or control on the actions of the organization.

Match the following terms and definitions:

supersets k entity cluster structural assertion subtype a specialization d anchor object action subtype discriminator attribute inheritance b overlap rule corresponding object e g derived fact

Contrast the following terms:

•  Supersets; subtype. A supersets is a generalized entity type that has
  

one or more subtypes, while a subtype is a suborning of the entities in a supersets.

With the overlap rule an instance of a supersets may simultaneously be a member of two or more subtypes.

• Structural assertion; action assertion. A structural assertion express the static structure of an organization whereas an action assertion is a statement of control on the actions of an organization.
• Total specialization rule; partial specialization rule. With the total specialization rule, each With the partial specialization rule, an instance of the supersets is allowed not to belong to any subtype.

Two conditions that indicate a designer should consider using supersets/subtype relationships:

• There are attributes that apply to some (but not all) of the instances of an entity type.
• The instances off subtype participate in the relationship unique to that subtype.

The reasons for entity clustering are:

1. Complex enterprise-wide E-R diagram.
2. Ability to have a hierarchical decomposition of data model.
3. Desire to focus part of the model on an area of interest to some community of users.
4. Ability to create several different entity clusters each with a different focus.

An example of a supersets/subtype

relationship: The supersets PERSON has many possible subtypes