This chapter extends the discussion of the basic ER model to some advanced concepts like generalization and specialization etc.

1. Learning Resource
On
Database Management Systems
Chapter-4
Enhanced Entity Relationship Model
Prepared By:
Kunal Anand, Asst. Professor
SCE, KIIT, DU, Bhubaneswar-24

2. Chapter Outcome:
• After the completion of this chapter, the students
will be able to:
– Define Enhanced Entity Relationship Model
– Distinguish between super class and sub class
– Distinguish between specialization and generalization
– Explain aggregation and composition
– Draw Enhanced ER Diagram
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3. Organization of this Chapter:
• Introduction
• Superclasses and Subclasses
• Specialization and Generalization
• Constraints on specialization/generalization
• Aggregation and Composition
• Sample example
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4. Enhanced ER Model
• The Enhanced ER model, EER model, includes all the
modeling concepts of ER model along with some additional
concepts like super class and sub class, specialization,
generalization, aggregation, and composition.
• Super class and Sub class:
– Super class is an entity type that has a relationship with one
or more subtypes. On the other hand, Sub class is a group
of entities with unique attributes.
– Sub class inherits properties and attributes from its super
class. Sub class and Super class relationship leads the
concept of Inheritance.
– The relationship between sub class and super class is
denoted with d symbol, where d means disjointness.
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5. contd..
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6. Specialization and Generalization
• Specialization
– The process of designating sub groupings within an
entity set is called Specialization.
– An entity set may be specialized by more than one
distinguishing features.
– ER-design, specialization is depicted by an
Inverted Triangle component labeled “IS A” (is a).
– Specialization can be repeatedly used to refine a
ER design.
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7. contd..
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8. EER Model (contd..)
• Generalization:
– Generalization is a simple inversion of specialization. It is
the process of extracting common properties from a set of
entity types and creating a generalized entity type from it.
– It is a bottom-up approach in which two or more entitiy
types can be generalized to a higher level entity type, if
they have some attributes in common.
– Specialization adopts top-down approach, while
Generalization adopts bottom-up approach.
– A crucial property of the higher-level and lower-level
entities created by specialization and generalization is
attribute inheritance.
– A lower-level entity set (or subclass) also inherits
participation in the relationship sets in which its higher-
level entity (or superclass) participates
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9. contd..
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10. Constraints on Generalization/Specialization
• Membership Constraint
– Condition defined
• Membership of a specialization/generalization relationship can be
defined as a condition in the requirements e.g. tanker is a ship where
cargo = oil.
– User-defined
• User-defined lower-level entity sets are not constrained by a
membership condition; rather, the database user assigns entities to a
given entity set.
• Disjoint constraints
– Disjoint:
• The disjoint constraint only applies when a superclass has more
than one subclass. If the subclasses are disjoint, then an entity
occurrence can be a member of only one of the subclasses.e.g.
postgrads or undergrads; one cannot be both.
• To represent a disjoint superclass/subclass relationship, Or is used.
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11. contd..
• Overlapping: This applies when an entity occurrence may be a
member of more than one subclass, e.g. student and staff; some
people are both. And is used to represent the overlapping
specialization/generalization relationship in the ER diagram.
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12. contd..
• Completeness constraints
– Total: Each superclass (higher-level entity) must belong to
subclasses (lower-level entity sets), e.g. a student must be
postgrad or undergrad. To represent completeness in the
specialization/generalization relationship, the keyword
Mandatory is used.
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13. contd..
• Partial: Some superclasses may not belong to subclasses
(lower-level entity sets), e.g. some people at UCT are neither
student nor staff. The keyword Optional is used to represent a
partial specialization/generalization relationship.
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14. Aggregation and Composition
• Aggregation:
– Aggregation represents a has-a relationship between entity types,
where one represents the whole and the other the part.
– An example of aggregation is the Car and Engine entities. A car is
made up of an engine. The car is the whole and the engine is the part.
– Aggregation does not represent strong ownership. This means, a part
can exist on its own without the whole. There is no stronger ownership
between a car and the engine. An engine of a car can be moved to
another car.
– A line with a diamond at the end is used to represent aggregation.The
whole must be put at the end of the diamond.
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15. contd..
• Composition
– Composition is a form of aggregation that represents an
association between entities, where there is a strong
ownership between the whole and the part.
– For example,: a tree and a branch have a composition
relationship. A branch is 'part' of a 'whole' tree - we cannot
cut the branch and add it to another tree.
– A line with a filled diamond at the end is used to represent
composition where the diamond side is towards the part
side.
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16. An Example
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