This document discusses the enhanced entity-relationship (EER) model. The EER model extends the basic ER model to represent more complex business data through the addition of supertypes and subtypes, generalization and specialization, constraints on supertype-subtype relationships, subtype discriminators, and entity clustering. Supertypes represent generic entity types that have subtypes as subgroups with both shared and distinct attributes. The EER model allows for more flexible modeling of real-world data.

1. Lecture 4:
Enhanced E-R Model
ISOM3260, Spring 2014

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Where we are now
• Database environment
– Introduction to database
• Database development process
– steps to develop a database
• Conceptual data modeling
– entity-relationship (ER) diagram; enhanced ER
• Logical database design
– transforming ER diagram into relations; normalization
• Physical database design
– technical specifications of the database
• Database implementation
– Structured Query Language (SQL), Advanced SQL
• Advanced topics
– data and database administration

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Database development activities during SDLC
Top-down
analysis

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Why Enhanced E-R Model?
• Business data are now more complex
• EER improve flexibility of the basic E-R
model to represent business data
• EER is the result of extending the original
E-R model with new modeling constructs

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Enhanced E-R Model
• Supertype and subtype
• Generalization and specialization
• Specifying constraints in supertype/subtype
relationships
• Supertype/subtype hierarchy
• Entity clustering

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Supertypes and Subtypes
• Subtype
– a subgrouping of the entities in an entity type, that is
meaningful to the organization, and which has
attributes or relationships that are distinct from those in
other subgroupings
– at the same time, the subgroupings share common
attributes which are associated with the entity type
– e.g. GRADUATE STUDENT and
UNDERGRADUATE STUDENT  subtypes of
STUDENT

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Supertypes and Subtypes
• Supertype
– a generic entity type that has a relationship with one
or more subtypes
– e.g. STUDENT  supertype of GRADUATE
STUDENT and UNDERGRADUATE STUDENT
• Attribute Inheritance
– subtype entities inherit values of all attributes
of the supertype
– an instance of a subtype is also an instance of
the supertype

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Figure 3-1: Basic notation for supertype/subtype relationships

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Figure 3-2: Employee supertype with three subtypes
All subtypes will have
Employee_Number,
Employee_Name, Address,
and Date_Hired
Each subtype will also
have its own attributes

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Supertype/Subtype Relationships
• Relationships at the supertype level
– indicate that all subtypes will participate in the
relationship
• Relationships at the subtype level
– the instances of a subtype may participate in a
relationship unique to that subtype

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Figure 3-3: Supertype/subtype relationships in a hospital
Both outpatients and
resident patients are cared
for by a responsible physician
Only resident patients
are assigned to a bed

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Generalization and Specialization
• Two processes to develop supertype/subtype
relationships
• Generalization
– the process of defining a more general entity type from a
set of more specialized entity types
– bottom-up process
• Specialization
– the process of defining one or more subtypes of the
supertype, and forming supertype/subtype relationships
– top-down process

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Figure 3-4a: Example of generalization
Three entity types: CAR, TRUCK, and MOTORCYCLE
All these types
of vehicles
have common
attributes

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Figure 3-4b: Generalization to VEHICLE supertype
So we put
the shared
attributes in
a supertype
No subtype for motorcycle, since it has no unique attributes

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Figure 3-5a: Example of specialization
Applies only to
manufactured parts
Applies only to
purchased parts
Parts may be manufactured internally and/or purchased from suppliers

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Figure 3-5b:
Specialization to MANUFACTURED PART and PURCHASED PART
Multivalued attribute “supplier” was replaced by a relationship to another entity
Created 2 subtypes

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Completeness Constraints in
Supertype/Subtype Relationships
• Completeness Constraint
– whether an instance of a supertype must also
be a member of at least one subtype
• Total Specialization Rule
– Yes (double line)
• Partial Specialization Rule
– No (single line)

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Figure 3-6: Examples of completeness constraints
(a) Total specialization rule
A patient must be either
an outpatient or a
resident patient

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Figure 3-6b: Partial specialization rule
A vehicle could be a car,
a truck, or neither

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Disjointness Constraints in
Supertype/Subtype Relationships
• Disjointness Constraint
– whether an instance of a supertype may simultaneously be a
member of two or more subtypes
• Disjoint Rule
– No (letter ‘d’)
– an instance of the supertype can be only ONE of the subtypes
• Overlap Rule
– Yes (letter ‘o’)
– an instance of the supertype can simultaneously be a member
of two or more subtypes

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(a) Disjoint rule
Figure 3-7: Examples of disjointness constraints
A patient can either be
outpatient or resident, but
not both, at any given time

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Figure 3-7b: Overlap rule
A part may be both
purchased and
manufactured

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Subtype Discriminators
• A mechanism to implement supertype/subtype
relationships
• Subtype Discriminator
– an attribute of the supertype whose values determine the
target subtype(s)
• Disjoint subtype
– a simple attribute with alternative values to indicate the
possible subtypes
• Overlapping subtype
– a composite attribute whose subparts pertain to different
subtypes
– each subpart contains a boolean value (‘Y’ or ‘N’) to indicate
whether or not the instance belongs to the associated subtype

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Figure 3-8: Introducing a subtype discriminator (disjoint rule)
A simple attribute with
different possible values
indicating the subtype

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Figure 3-9: Subtype discriminator (overlap rule)
A composite attribute
with sub-attributes
indicating “Y” or “N” to
determine whether it is
of each subtype

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Supertype/Subtype Hierarchies
• Attributes are assigned at the highest logical level in the
hierarchy
– e.g. SSN applies to all PERSONs
– e.g. Date_Hired applies only to EMPLOYEE
• Subtypes lower in the hierarchy inherit attributes from
their supertypes up to the root (entity type at top of
hierarchy)
– e.g. an instance of FACULTY has the following attributes:
– SSN, Name, Address, Gender, Date_of_Birth (from PERSON)
– Date_Hired, Salary (from EMPLOYEE)
– Rank (from FACULTY)

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Figure 3-10: Example of supertype/subtype hierarchy

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Entity Clusters
• EER diagrams are difficult to read when there are
too many entities and relationships
• Different people may be interested in different
types of data
• Solution
– group entities and relationships into entity clusters
• Entity cluster
– a set of one or more entity types and associated
relationships grouped into a single abstract entity type

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Figure 3-13a: Possible entity clusters for Pine Valley Furniture
Related groups
of entities can
become clusters

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Figure 3-13b: EER diagram of entity clusters
More readable,
isn’t it?

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Figure: ER model for class scheduling

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Review Questions
• What are subtypes and supertypes?
• What are generalization and specialization?
• What are the total and partial specialization rules?
• What are the disjoint and overlap rules?
• What are subtype discriminators?
• What are supertype/subtype hierarchies?
• What are entity clusters?