Chapter04

Copyright © 2007 Ramez Elmasri and Shamkant B. Navathe
Chapter 4
Enhanced Entity-Relationship
(EER) Modeling

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

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

Subclasses and Superclasses

 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
 …

 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

 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

Representing Specialization in EER
Diagrams

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

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

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

Specialization (3)

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

Generalization (2)

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

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

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

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

Displaying an attribute-defined
specialization in EER diagrams

Generalization (3)
 Two basic constraints can apply to a
specialization/generalization:
 Disjointness Constraint:
 Completeness Constraint:

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

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

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.

Example of disjoint partial Specialization

Example of overlapping total Specialization

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)

Shared Subclass “Engineering_Manager”

 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)

 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

Specialization / Generalization Lattice
Example (UNIVERSITY)

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

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

Two categories (UNION types):
OWNER, REGISTERED_VEHICLE

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

 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.

 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.

 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])

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

UML Example for Displaying
Specialization / Generalization

Alternative Diagrammatic Notations

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)

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

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

Chapter04

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