Enhanced entity-relationship model
Chapter 8
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Enhanced ER model
EER model = concepts of ER model &
{super class, subclass, specialization, generalization}
• The EER model includes all the modeling concepts of the ER model. In
addition, it includes the concepts of subclass and superclass and the related
concepts of specialization and generalization.
• Another concept included in the EER model is that of a category or union type
which is used to represent a collection of objects (entities) that is the union of
objects of different entity types.
• Associated with these concepts is the important mechanism of attribute and
relationship inheritance.
• The database schema obtained through EER modeling is called EER schema.
Pictorial representation of EER schema is called EER diagram.
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EER model
EER model incorporates more accuracy into a
database design, if required.
Application of EER model
i) Engineering design and manufacturing (CAD/CAM)
ii) Telecommunications
iii) Complex software systems &
iv) Geographic Information Systems (GIS)
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Specialization
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Specialization
Specialization is a process of defining a set of subclasses of an
entity type.
superclass/subclass relationship
e.g. EMPLOYEE/SECRETARY,
EMPLOYEE/ENGINEER,
EMPLOYEE/MANAGER,
EMPLOYEE/SALARIED_EMPLOYEE
Different from 1:1 relationship type
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An attribute-defined specialization
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Specialization
Specialization allows us to do the following:
• Define a set of subclasses of an entity type
• Specify additional attributes of each subclass
• Establish additional specific relationship types between each
subclass and other entity types or other subclasses.
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Generalization
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Generalization
Generalization defines a generalized entity type from a
number of entity types.
Generalization process may be viewed as being
functionally inverse of specialization.
Generalization is a process of synthesis.
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Constraints on Specialization & Generalization
Predicate-defined specialization –
Membership of an entity in a sub class of a specialization is defined by
a predicate
Attribute-defined specialization – membership of an entity in a sub
class of a specialization is defined by value of a single attribute (called
defining attribute)
User-defined specialization – membership of an entity in a sub class
is defined by the database user.
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Predicate-defined specialization
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Attribute-defined specialization
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Constraints on
Specialization & Generalization
Disjoint specialization & Overlapping specialization
Partial specialization &Total specialization
• A specialization is called disjoint specialization if two or more subclasses
do contain the same entity otherwise it is called overlapping.
• A specialization is called partial if it is not necessary for every entity of a
superclass to find its place in one of the subclasses otherwise the
specialization is called total specialization.
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Overlapping specialization
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Constraints on
specialization & generalization
Disjointedness & completeness are independent
•
•
•
•
Disjoint and total
Disjoint and partial
Overlapping and total
Overlapping and partial
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Specialization Lattice
ENGINEERING_MANAGER is a Shared subclass
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Specialization Lattice
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EER model
Specialization lattice – one sub class with multiple super
classes (shared sub class).
Specialization hierarchy – one sub class with one super
class only.
Existence of at least one shared subclass leads to a lattice.
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Specialization vs. Generalization
•
•
•
•
Specialization is a process of decomposition
Generalization is a process of synthesis
Specialization maximizes differences among entity types
Generalization minimizes differences among entity types
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EER model – Union type/ Category
Data requirements for a DB for motor vehicle registration.
• The owner of a registered vehicle is either a person or a
company or a bank.
• A registered vehicle can be either a car or a truck.
• Only registered vehicles bear license plate number. A license
plate number can uniquely determine a registered vehicle.
• Vehicles that are not registered have vehicle id as its key.
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Entity types – PERSON, COMPANY & BANK
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Union type/category
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Entity types – CAR & TRUCK
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Union type/category
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EER diagram
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Union type/Category
• A union type/category is a sub class of union of multiple distinct entity
types
• A union type has two or more super classes that may represent distinct
entity types
• A union type is a subset of the union of its super classes
• Selective attribute inheritance
• Total / partial union type
• OWNER is a category, REGISTERED_VEHICLE is a category
• Super classes of a category may have different key attributes
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A Category may be replaced by a specialization
 If category is total consider representing it using
specialization.
 If the two classes represent the same type of
entities and share numerous attributes, including the same
key attributes, specialization / generalization is preferred;
 otherwise, category (union type) is more appropriate.
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Design choices in EER
• Union type should be avoided.
• Do not create too many sub classes.
• Sub classes with small number of local attributes should be merged with
super class.
• Create a sub class when it is independently associated in a relationship
with other entities the super class do not participate in.
• Default choice for disjoint/overlapping, total/partial are overlapping &
partial.
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An example
The following narrative describes a simplified version of the
organization of Olympic facilities planned for the summer
Olympics.
Draw an EER diagram that shows the entity types, attributes,
relationships, and specializations for this application. State any
assumptions you make.
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The Olympic facilities are divided into sports complexes. Sports
complexes are divided into one-sport and multi sport types.
Multi sport complexes have areas of the complex designated for
each sport with a location indicator (e.g. center, NE corner and
soon). A complex has a location, chief organizing individual, total
occupied area and so on.
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Each complex holds a series of events (e.g. the track stadium
may hold many different races). For each event there is a
planned date, duration, number of participants, number of
official and so on.
A roster of all officials will be maintained together with the list of
events each official will be involved in.
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Different equipment is needed for the events (e.g. goal posts,
poles, parallel bars) as well as for maintenance. The two types of
facilities (one-sport and multi sport) will have different types of
information. For each type, the number of facilities needed is
kept, together with an approximate budget.
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Museum Database
Design a database to keep track of information for an art
museum. Assume that the following data requirements were
collected:
The museum has a collection of ART_OBJECTs. Each ART_OBJECT
has a unique Id_no, an Artist (if known), a Year (when it was
created, if known), a Title, and a Description. The art objects are
categorized in several ways, as discussed below.
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ART_OBJECTs are categorized based on their type. There are three main
types: PAINTING, SCULPTURE, and STATUE, plus another type called OTHER to
accommodate objects that do not fall into one of the three main types.
A PAINTING has a Paint_type (oil, watercolor, etc.), material on which it is
Drawn_on (paper, canvas, wood, etc.), and Style (modern, abstract, etc.).
A SCULPTURE or a statue has a Material from which it was created (wood,
stone, etc.), Height, Weight, and Style.
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An art object in the OTHER category has a Type (print, photo, etc.) and Style.
ART_OBJECTs are categorized as either PERMANENT_COLLECTION (objects that
are owned by the museum) and BORROWED.
Information captured about objects in the PERMANENT_COLLECTION includes
Date_acquired, Status (on display, on loan, or stored), and Cost.
Information captured about BORROWED objects includes the collection from
which it was borrowed, Date_borrowed, and Date_returned.
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Information describing the country or culture of Origin (Italian, Egyptian,
American, Indian, and so forth) and Epoch (Renaissance, Modern, Ancient,
and so forth) is captured for each ART_OBJECT.
The museum keeps track of ARTIST information, if known: Name, DateBorn (if
known), Date_died (if not living), Country_of_origin, Epoch, Main_style, and
Description. The Name is assumed to be unique.
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Different EXHIBITIONS occur, each having a Name, Start_date, and End_date.
EXHIBITIONS are related to all the art objects that were on display during the
exhibition.
Information is kept on other COLLECTIONS with which the museum interacts,
including Name (unique), Type (museum, personal, etc.), Description,
Address, Phone, and current Contact_person.
Draw an EER schema diagram for this application. Discuss any assumptions
you make, and that justify your EER design choices.
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