CH4 The Enhanced Entity–Relationship (EER) Model

Question 1

4.1. What is a subclass? When is a subclass needed in data modeling?

Answer 1

• The set or collection of entities in each of the groupings is a subset of the entities that belong to the superclass entity set, meaning that every entity that is a member of one of these subgroupings is also a member of the superclass
• subgroupings or subtypes of its entities that are meaningful and need to be represented explicitly because of their significance to the database application
• a member entity of the subclass represents the same real-world entity as some member of the superclass
• a subclass inherits all the attributes of the entity as a member of the superclass. The entity also inherits all the relationships in which the superclass participates. N

Question 2

Define the following terms: superclass of a subclass,

Answer 2

- We call each of these subgroupings a
subclass or subtype of the EMPLOYEE entity type, and the EMPLOYEE entity type is called the superclass or supertype for each of these subclasses

Question 3

Define the following terms: superclass/subclass rela- tionship

Answer 3

the relationship between a superclass and any one of its subclasses

Question 4

Define specialization

Answer 4

• Specialization is the process of defining a set of subclasses of an entity type
• e set of subclasses that forms a specialization is defined on the basis of some distinguishing characteristic
• A specialization Z = {S1, S2, … , Sn} is a set of subclasses that have the same super- class G;

Question 5

Define generalizatio

Answer 5

• We use the term generalization to refer to the pro- cess of defining a generalized entity type from the given entity types
• G is called a generalized entity type (or the superclass of the specialization, or a generalization of the subclasses {S1, S2, … , Sn} )

Question 6

What is the difference between a specialization hierarchy and a specializa-
tion lattice?

Answer 6

A specialization hierarchy has the constraint that every subclass participates as a subclass in only one class/subclass relationship; that is, each subclass has only one parent, which results in a tree structure or strict hierarchy. In contrast, for a specialization lattice, a subclass can be a subclass in more than one class/subclass relationship.

Question 7

How does a category differ from a regular shared subclass? What is a cate-
gory used for? Illustrate your answer with examples.

Answer 7

• a subclass will represent a collection of entities that is a subset of the UNION of entities from distinct entity types; we call such a subclass a union type or category
• A category has two or more superclasses that may represent collections of enti- ties from distinct entity types, whereas other superclass/subclass relationships always have a single superclass. To
• In a category the entity must exist in only one of the superclasses
• In a shared class the entity set is a subset of the intersection of the superclass entity sets.
• category OWNER entity inherits the attributes of a COMPANY, a PERSON, or a BANK, depending on the superclass to which the entity belongs. On the other hand, a shared subclass such as ENGINEERING_MANAGER inherits all the attributes of its superclasses SALARIED_EMPLOYEE, ENGINEER, and MANAGER.

Question 8

Define class

Answer 8

A class11 defines a type of entity and represents a set or collection of entities of that type

Question 9

What are the main similarities and differences between conceptual database modeling techniques and knowledge representation techniques?

Answer 9

Similarities

• identify common properties and important aspects of objects in the miniworld
• concepts, relationships, constraints, operations, and languages for defining data and representing knowledge

Differences
- Knowledge has reasoning mechanisms and can inference

Question 10

Discuss the similarities and differences between an ontology and a database schema.

Answer 10

e main difference between an ontology and, say, a database schema, is that the schema is usually limited to describing a small subset of a mini- world from reality in order to store and manage data. An ontology is usually con- sidered to be more general in that it attempts to describe a part of reality or a domain of interest (for example, medical terms, electronic-commerce applications, sports, and so on) as completely as possib

Question 11

Subclass EER diagram convention

Answer 11

• rectangle

- Fork pointing to the superclass. Connect superclass and subclass

Question 12

specialization EER diagram convention

Answer 12

The subclasses that define a specialization are attached by lines to a circle
that represents the specialization,

Question 13

predicate-defined subclass EER diagram convention

Answer 13

predicate-defined subclass by writing the predicate condition next to the line that connects the subclass to the specialization circle.

Question 14

4.4. Discuss user-defined and predicate-defined subclasses, and identify the dif- ferences between the two.

Answer 14

• When we do not have a condition for determining membership in a subclass, the subclass is called user-defined. Membership in such a subclass is determined by the database users when they apply the operation to add an entity to the subclass; h
• exactly the entities that will become members of each subclass by placing a condition on the value of some attribute of the superclass. Such subclasses are called predicate-defined

Question 15

specialization disjoint EER diagram convention

Answer 15

d in the circle stands for disjoint.

Question 16

overlapping (nondisjoint) specialization EER diagram convention

Answer 16

o in the circle stands for disjoint.for an overlapping (nondisjoint) specialization.

Question 17

4.5. Discuss user-defined and attribute-defined specializations, and identify the differences between the two.

Answer 17

- Keyword: all. If all subclasses in a specialization have their membership condition on the same
attribute of the superclass, the specialization itself is called an attribute-defined
specialization
- A specialization that is attribute-defined implies the disjointness constraint 
- When we do not have a condition for determining membership in a subclass, the subclass is called user-defined. Membership in such a subclass is determined by the database users when they apply the operation to add an entity to the subclass; h

Question 18

4.6. Discuss the two main types of constraints on specializations and generalizations.

Answer 18

• The two main constraints are total/partial and disjoint/overlapping.
• subclasses of the specialization must be disjoint sets. This means that an entity can be a member of at most one of the subclasses of the specialization
• sets may be overlapping; that is, the same (real-world) entity may be a member of more than one subclass of the specialization
• total specialization constraint specifies
  that every entity in the superclass must be a member of at least one subclass
  in the specializatio
• partial specialization,
  which allows an entity not to belong to any of the subclasses

Question 19

total specialization EER diagram convention

Answer 19

a double line to connect
the superclass to the circle

Question 20

category EER diagram convention

Answer 20

The superclasses COMPANY, BANK, and PERSON are connected to the circle with the ∪ symbol, which stands for the set union operation

Question 21

Category types

Answer 21

A category can be total or partial. A total category holds the union of all entities in its superclasses, whereas a partial category can hold a subset of the union. A

Question 22

total category, partial EER diagram convention

Answer 22

A total category is represented diagrammatically by a double line connecting the category and the circle, whereas a partial category is indicated by a single line.

Question 23

Define specific (local) attributes

Answer 23

• Attributes that apply only to entities of a particular subclass
• Keyword: only

Question 24

Define specific relationships.

Answer 24

• relationship types that apply only to entities of a particular subclass
• Keyword: only

Question 25

it is not necessary that every entity in a superclass is a member of some subclass.

true/false

Answer 25

true

Question 26

Why use subclasses?

Answer 26

• The first is that certain attributes may apply to some but not all entities in the superclass
• The second reason for using subclasses is that some relationship types may be par- ticipated in only by entities that are members of the subclass