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Chapter 4

Enhanced Entity-Relationship (EER) Modeling

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Chapter Outline

EER stands for Enhanced ER or Extended ER
EER Model Concepts

Includes all modeling concepts of basic ER
Additional concepts:

subclasses/superclasses
specialization/generalization
categories (UNION types)
attribute and relationship inheritance

These are fundamental to conceptual modeling

The additional EER concepts are used to model applications more completely and more accurately

EER includes some object-oriented concepts, such as inheritance

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Subclasses and Superclasses (1)

An entity type may have additional meaningful subgroupings of its entities

Example: EMPLOYEE may be further grouped into:

SECRETARY, ENGINEER, TECHNICIAN, …

Based on the EMPLOYEE’s Job

MANAGER

EMPLOYEEs who are managers

SALARIED_EMPLOYEE, HOURLY_EMPLOYEE

Based on the EMPLOYEE’s method of pay

EER diagrams extend ER diagrams to represent these additional subgroupings, called subclasses or subtypes

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Subclasses and Superclasses

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Subclasses and Superclasses (2)

Each of these subgroupings is a subset of EMPLOYEE entities
Each is called a subclass of EMPLOYEE
EMPLOYEE is the superclass for each of these subclasses
These are called superclass/subclass relationships:

EMPLOYEE/SECRETARY
EMPLOYEE/TECHNICIAN
EMPLOYEE/MANAGER
…

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Subclasses and Superclasses (3)

These are also called IS-A relationships

SECRETARY IS-A EMPLOYEE, TECHNICIAN IS-A EMPLOYEE, ….

Note: An entity that is member of a subclass represents the same real-world entity as some member of the superclass:

The subclass member is the same entity in a distinct specific role
An entity cannot exist in the database merely by being a member of a subclass; it must also be a member of the superclass
A member of the superclass can be optionally included as a member of any number of its subclasses

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Subclasses and Superclasses (4)

Examples:

A salaried employee who is also an engineer belongs to the two subclasses:

ENGINEER, and
SALARIED_EMPLOYEE

A salaried employee who is also an engineering manager belongs to the three subclasses:

MANAGER,
ENGINEER, and
SALARIED_EMPLOYEE

It is not necessary that every entity in a superclass be a member of some subclass

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Representing Specialization in EER Diagrams

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Attribute Inheritance in Superclass / Subclass Relationships

An entity that is member of a subclass inherits

All attributes of the entity as a member of the superclass
All relationships of the entity as a member of the superclass

Example:

In the previous slide, SECRETARY (as well as TECHNICIAN and ENGINEER) inherit the attributes Name, SSN, …, from EMPLOYEE
Every SECRETARY entity will have values for the inherited attributes

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Specialization (1)

Specialization is the process of defining a set of subclasses of a superclass
The set of subclasses is based upon some distinguishing characteristics of the entities in the superclass

Example: {SECRETARY, ENGINEER, TECHNICIAN} is a specialization of EMPLOYEE based upon job type.

May have several specializations of the same superclass

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Specialization (2)

Example: Another specialization of EMPLOYEE based on method of pay is {SALARIED_EMPLOYEE, HOURLY_EMPLOYEE}.

Superclass/subclass relationships and specialization can be diagrammatically represented in EER diagrams
Attributes of a subclass are called specific or local attributes.

For example, the attribute TypingSpeed of SECRETARY

The subclass can also participate in specific relationship types.

For example, a relationship BELONGS_TO of HOURLY_EMPLOYEE

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Specialization (3)

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Generalization

Generalization is the reverse of the specialization process
Several classes with common features are generalized into a superclass;

original classes become its subclasses

Example: CAR, TRUCK generalized into VEHICLE;

both CAR, TRUCK become subclasses of the superclass VEHICLE.
We can view {CAR, TRUCK} as a specialization of VEHICLE
Alternatively, we can view VEHICLE as a generalization of CAR and TRUCK

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Generalization (2)

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Generalization and Specialization (1)

Diagrammatic notation are sometimes used to distinguish between generalization and specialization

Arrow pointing to the generalized superclass represents a generalization
Arrows pointing to the specialized subclasses represent a specialization
We do not use this notation because it is often subjective as to which process is more appropriate for a particular situation
We advocate not drawing any arrows

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Generalization and Specialization (2)

Data Modeling with Specialization and Generalization

A superclass or subclass represents a collection (or set or grouping) of entities
It also represents a particular type of entity
Shown in rectangles in EER diagrams (as are entity types)
We can call all entity types (and their corresponding collections) classes, whether they are entity types, superclasses, or subclasses

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Constraints on Specialization and Generalization (1)

If we can determine exactly those entities that will become members of each subclass by a condition, the subclasses are called predicate-defined (or condition-defined) subclasses

Condition is a constraint that determines subclass members
Display a predicate-defined subclass by writing the predicate condition next to the line attaching the subclass to its superclass

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Constraints on Specialization and Generalization (2)

If all subclasses in a specialization have membership condition on same attribute of the superclass, specialization is called an attribute-defined specialization

Attribute is called the defining attribute of the specialization
Example: JobType is the defining attribute of the specialization {SECRETARY, TECHNICIAN, ENGINEER} of EMPLOYEE

If no condition determines membership, the subclass is called user-defined

Membership in a subclass is determined by the database users by applying an operation to add an entity to the subclass
Membership in the subclass is specified individually for each entity in the superclass by the user

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Displaying an attribute-defined specialization in EER diagrams

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Constraints on Specialization and Generalization (3)

Two basic constraints can apply to a specialization/generalization:

Disjointness Constraint:
Completeness Constraint:

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Constraints on Specialization and Generalization (4)

Disjointness Constraint:

Specifies that the subclasses of the specialization must be disjoint:

an entity can be a member of at most one of the subclasses of the specialization

Specified by d in EER diagram
If not disjoint, specialization is overlapping:

that is the same entity may be a member of more than one subclass of the specialization

Specified by o in EER diagram

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Constraints on Specialization and Generalization (5)

Completeness Constraint:

Total specifies that every entity in the superclass must be a member of some subclass in the specialization/generalization
Shown in EER diagrams by a double line
Partial allows an entity not to belong to any of the subclasses
Shown in EER diagrams by a single line

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Constraints on Specialization and Generalization (6)

Hence, we have four types of specialization/generalization:

Disjoint, total
Disjoint, partial
Overlapping, total
Overlapping, partial

Note: Generalization usually is total because the superclass is derived from the subclasses.

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Example of disjoint partial Specialization

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Example of overlapping total Specialization

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Specialization/Generalization Hierarchies, Lattices & Shared Subclasses (1)

A subclass may itself have further subclasses specified on it

forms a hierarchy or a lattice

Hierarchy has a constraint that every subclass has only one superclass (called single inheritance); this is basically a tree structure
In a lattice, a subclass can be subclass of more than one superclass (called multiple inheritance)

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Shared Subclass “Engineering_Manager”

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Specialization/Generalization Hierarchies, Lattices & Shared Subclasses (2)

In a lattice or hierarchy, a subclass inherits attributes not only of its direct superclass, but also of all its predecessor superclasses
A subclass with more than one superclass is called a shared subclass (multiple inheritance)
Can have:

specialization hierarchies or lattices, or
generalization hierarchies or lattices,
depending on how they were derived

We just use specialization (to stand for the end result of either specialization or generalization)

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Specialization/Generalization Hierarchies, Lattices & Shared Subclasses (3)

In specialization, start with an entity type and then define subclasses of the entity type by successive specialization

called a top down conceptual refinement process

In generalization, start with many entity types and generalize those that have common properties

Called a bottom up conceptual synthesis process

In practice, a combination of both processes is usually employed

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Specialization / Generalization Lattice Example (UNIVERSITY)

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Categories (UNION TYPES) (1)

All of the superclass/subclass relationships we have seen thus far have a single superclass
A shared subclass is a subclass in:

more than one distinct superclass/subclass relationships
each relationships has a single superclass
shared subclass leads to multiple inheritance

In some cases, we need to model a single superclass/subclass relationship with more than one superclass
Superclasses can represent different entity types
Such a subclass is called a category or UNION TYPE

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Categories (UNION TYPES) (2)

Example: In a database for vehicle registration, a vehicle owner can be a PERSON, a BANK (holding a lien on a vehicle) or a COMPANY.

A category (UNION type) called OWNER is created to represent a subset of the union of the three superclasses COMPANY, BANK, and PERSON
A category member must exist in at least one of its superclasses

Difference from shared subclass, which is a:

subset of the intersection of its superclasses
shared subclass member must exist in all of its superclasses

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Two categories (UNION types): OWNER, REGISTERED_VEHICLE

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Formal Definitions of EER Model (1)

Class C:

A type of entity with a corresponding set of entities:

could be entity type, subclass, superclass, or category

Note: The definition of relationship type in ER/EER should have 'entity type' replaced with 'class‘ to allow relationships among classes in general
Subclass S is a class whose:

Type inherits all the attributes and relationship of a class C
Set of entities must always be a subset of the set of entities of the other class C

S ⊆ C

C is called the superclass of S
A superclass/subclass relationship exists between S and C

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Formal Definitions of EER Model (2)

Specialization Z: Z = {S1, S2,…, Sn} is a set of subclasses with same superclass G; hence, G/Si is a superclass relationship for i = 1, …., n.

G is called a generalization of the subclasses {S1, S2,…, Sn}
Z is total if we always have:

S1 ∪ S2 ∪ … ∪ Sn = G;
Otherwise, Z is partial.

Z is disjoint if we always have:

Si ∩ S2 empty-set for i ≠ j;

Otherwise, Z is overlapping.

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Formal Definitions of EER Model (3)

Subclass S of C is predicate defined if predicate (condition) p on attributes of C is used to specify membership in S;

that is, S = C[p], where C[p] is the set of entities in C that satisfy condition p

A subclass not defined by a predicate is called user-defined
Attribute-defined specialization: if a predicate A = ci (where A is an attribute of G and ci is a constant value from the domain of A) is used to specify membership in each subclass Si in Z

Note: If ci ≠ cj for i ≠ j, and A is single-valued, then the attribute-defined specialization will be disjoint.

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Formal Definitions of EER Model (4)

Category or UNION type T

A class that is a subset of the union of n defining superclasses �D1, D2,…Dn, n>1:

T ⊆ (D1 ∪ D2 ∪ … ∪ Dn)

Can have a predicate pi on the attributes of Di to specify entities of Di that are members of T.
If a predicate is specified on every Di: T = (D1[p1] ∪ D2[p2] ∪…∪ Dn[pn])

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Alternative diagrammatic notations

ER/EER diagrams are a specific notation for displaying the concepts of the model diagrammatically
DB design tools use many alternative notations for the same or similar concepts
One popular alternative notation uses UML class diagrams
see next slides for UML class diagrams and other alternative notations

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UML Example for Displaying Specialization / Generalization

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Alternative Diagrammatic Notations

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General Conceptual Modeling Concepts

GENERAL DATA ABSTRACTIONS

CLASSIFICATION and INSTANTIATION
AGGREGATION and ASSOCIATION (relationships)
GENERALIZATION and SPECIALIZATION
IDENTIFICATION

CONSTRAINTS

CARDINALITY (Min and Max)
COVERAGE (Total vs. Partial, and Exclusive (disjoint) vs. Overlapping)

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Ontologies

Use conceptual modeling and other tools to develop “a specification of a conceptualization”

Specification refers to the language and vocabulary (data model concepts) used
Conceptualization refers to the description (schema) of the concepts of a particular field of knowledge and the relationships among these concepts

Many medical, scientific, and engineering ontologies are being developed as a means of standardizing concepts and terminology

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Summary

Introduced the EER model concepts

Class/subclass relationships
Specialization and generalization
Inheritance

These augment the basic ER model concepts introduced in Chapter 3
EER diagrams and alternative notations were presented

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