Enhanced Entity-Relationship Modeling

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Chapter 12

• Enhanced Entity-Relationship Modeling

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Enhanced Entity-Relationship Model

• Basic concepts of ER modeling are not sufficient
  to represent requirements of newer, more complex
  applications.
• Semantic concepts are incorporated into the
  original ER model and called the Enhanced
  Entity-Relationship (EER) model.
• Examples of additional concepts of EER model are
• specialization / generalization
• aggregation

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Specialization / Generalization

• Superclass
• An entity type that includes one or more distinct
  subgroupings of its occurrences.
• Subclass
• A distinct subgrouping of occurrences of an
  entity type.

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Specialization / Generalization

• Superclass/subclass relationship is one-to-one
  (11).
• Superclass may contain overlapping or distinct
  subclasses.
• Not all members of a superclass need be a member
  of a subclass.

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Specialization / Generalization

• Attribute Inheritance
• An entity in a subclass may possess
  subclass-specific attributes, as well as those
  associated with the superclass.
• Specialization Process of maximizing differences
  between members of an entity by identifying their
  distinguishing characteristics.
• Generalization Process of minimizing differences
  between entities by identifying their common
  characteristics. .

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AllStaff relation holding details of all staff
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Specialization/generalization of Staff entity
into subclasses representing job roles
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Specialization/generalization of Staff entity
into job roles and contracts of employment
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EER diagram with shared subclass and subclass
with its own subclass
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Constraints on Specialization / Generalization

• Two constraints that may apply to a
  specialization/generalization
• participation and disjoint constraints
• Participation constraint Determines whether
  every member in superclass must participate as a
  member of a subclass.
• May be mandatory or optional.
• Disjoint constraint Describes whether member of
  a superclass can be a member of one, or more than
  one, subclass.
• May be disjoint (Or) or nondisjoint (And).

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Constraints on Specialization / Generalization

• There are four categories of constraints of
  specialization and generalization
• mandatory and disjoint
• optional and disjoint
• mandatory and nondisjoint
• optional and nondisjoint.
• Reasons for using Specialization/Generalization
• To add descriptive attributes specific to a
  subclass
• To identify entities that participate in a
  relationship

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DreamHome worked example - Staff Superclass with
Supervisor and Manager subclasses
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DreamHome worked example - Owner Superclass with
PrivateOwner and BusinessOwner subclasses
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EER diagram of Branch view of DreamHome with
specialization/generalization
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Aggregation (textbook uses a different concept)

• Different meaning in UML (has-a or is-part-of
  relationships in UML)
• Used when we have to model a relationship
  involving a relationship type
• Aggregation allows us to treat a relationship
  type as a high level entity for purpose of
  participation in other relationships.

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Example of Aggregation
Contact_Name Contact_Phone Date
Job_Applicant SSN Name Address Phone
Company Cname Caddress
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Interview
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Starting_Date Starting_Salary
Results_in
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Job_Offer Position
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Example of incorrect design
Job_Applicant SSN Name Address Phone
Company Cname Caddress
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Contact_Name Contact_Phone Date Starting_Date Star
ting_Salary
If an interview does not result in an offer, then
Starting_Date and Starting_Salary are
NULL. Multiplicities are different than those on
previous diagram. May show a wrong interview
instance that is not allowed by the aggregation.
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Job_Offer Position