Chapter04 database system in computer.ppt

CHAPTER 4
Enhanced Entity-Relationship
(EER) Modeling
Slide 1- 2

Copyright © 2016 Ramez Elmasri and Shamkant B. Navathe Slide 4- 3
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

Constraints on Specialization/Generalization
 The additional EER concepts are used to model
applications more completely and more accurately

EER includes some object-oriented concepts, such as
inheritance
 Knowledge Representation and Ontology Concepts

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 (the role they play)

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.

Example: MANAGER is a specialization of
EMPLOYEE based on the role the employee plays

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 notations 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

Types of Specialization
 Predicate-defined ( or condition-defined) : based
on some predicate. E.g., based on value of an
attribute, say, Job-type, or Age.
 Attribute-defined: shows the name of the attribute
next to the line drawn from the superclass toward
the subclasses (see Fig. 4.1)
 User-defined: membership is defined by the user
on an entity by entity basis
Slide 4- 18

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 (Exhaustiveness) 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 (typically just
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

Knowledge Representation (KR)-1
 Deals with modeling and representing a certain
domain of knowledge.
 Typically done by using some formal model of
representation and by creating an Ontology
 An ontology for a specific domain of interest
describes a set of concepts and interrelationships
among those concepts
 An Ontology serves as a “schema” which enables
interpretation of the knowledge in a “knowledge-
base”
Slide 4- 43

COMMON FEATURES between KR and Data Models:
 Both use similar set of abstractions – classification,
aggregation, generalization, and identification.
 Both provide concepts, relationships, constraints,
operations and languages to represent knowledge and
model data
DIFFERENCES:
 KR has broader scope: tries to deal with missing and
incomplete knowledge, default and common-sense
knowledge etc.
Slide 4- 44

DIFFERENCES (continued):
KR schemes typically include rules and reasoning
mechanisms for inferencing
Most KR techniques involve data and metadata. In
data modeling, these are treated separately
KR is used in conjunction with artificial intelligence
systems to do decision support applications
For more details on spatial, temporal and multimedia data
modeling, see Chapter 26. For details on use of Ontologies
see Sections 27.4.3 and 27.7.4.
Slide 4- 45

General Basis for Conceptual Modeling
 TYPES OF 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
 Constraints on EER schemas
 These augment the basic ER model concepts introduced
in Chapter 3
 EER diagrams and alternative notations were presented
 Knowledge Representation and Ontologies were
introduced and compared with Data Modeling

Chapter04 database system in computer.ppt

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