Chapter 3: ER and EER Models

Question 1

What are the three building blocks of an Entity Relationship Model?

Answer 1

Entity Types, Attribute Types, and Relationship Types

Question 2

What does ER Model stand for?

Answer 2

Entity Relationship Model

Question 3

What is an Entity Type?

Answer 3

An entity type is a “thing”, a noun, something that has an unambiguous meaning. Examples include supplier, customer, product, student, etc.

Question 4

What is the difference between Entity Type and Entity?

Answer 4

An Entity Type is the heading of the table, basically what type of thing (entity) it will contain (ex. Student). An entity is an instance in that table (ex. John Smith)

Question 5

How is Entity Type depicted in an ER model?

Answer 5

Using a rectangle

Question 6

What is an Attribute Type?

Answer 6

An attribute type is a property of an entity, it is something about the “thing”. Examples for “Customer” include name, address, customerID, status

Question 7

How is an Attribute Type depicted in an ER model?

Answer 7

Using an ellipses

Question 8

What is a Domain?

Answer 8

A Data Domain refers to all the valid values which a data element (column) may contain. You can’t represent domain using an ER model (Ex. gender may only be male or female)

Question 9

What is a Key Attribute Type?

Answer 9

A Key Attribute Type is used to denote the property that uniquely identifies an entity (customerID), and which is mapped to the Primary Key field in a database

Question 10

What is the difference between a Simple and Composite Attribute Type?

Answer 10

A Simple Attribute Type cannot be further divided into parts. Examples include supplier numbers or supplier status.

A Composite Attribute Type is one which can be broken down into sup-parts. Examples include address being broken down further into street, number, city, postal code, country, etc.

Question 11

What is the difference between Single-Valued and Multi-Valued Attribute Types and how are they depicted in an ER model?

Answer 11

A Single-Valued Attribute Type is one that can only have one value, for example, supplierID. A Multi-Valued Attribute Type is one that can have multiple values, for example, multiple email addresses for one supplier.

A Multi-Valued Attribute Type is depicted using a double ellipses

Question 12

What is a Derived Attribute?

Answer 12

A Derived Attribute is one that is derived from another attribute. An example is Age being derived using the difference between the attribute type Date of Birth and the current date

Question 13

How is a Relationship Type depicted in an ER model?

Answer 13

Using a rhombus shape (diamond)

Question 14

What is the Degree of a relationship type?

Answer 14

A Degree is the number of entity types participating in the relationship type

Question 15

What is Cardinality of relationship types?

Answer 15

Cardinality specifies the minimum or maximum number of relationship instances that an individual entity can participate in

Question 16

What are the Minimum Cardinalities and what do they mean?

Answer 16

Can either be 0 or 1.

0 means that the entity can occur without being connected through a relationship to another entity

1 means that the entity must always be connected through a relationship to another entity

Question 17

What are the Maximum Cardinalities and what do they mean?

Answer 17

Can either be 1 or N.

1 means the entity can only be in a relationship with one other entity

N means the entity can be in a relationship with N entities at once

Question 18

What does 1:1 mean?

Answer 18

Entity A can only be assigned to at most one other entity (a student can have no thesis or one thesis), and Entity B can only be assigned to at most one other entity (a thesis can have no student or one student)

Question 19

What does 1:N mean?

Answer 19

Entity A can only be assigned to at most one other entity (a project must and can only be managed by one person), but Entity B can be assigned to many other entities at once (a person can manage zero or many projects)

Question 20

What does N:1 mean?

Answer 20

Entity A can be assigned to many other entities at once, but Entity B can only be assigned to at most one other entity

Question 21

What does N:M mean?

Answer 21

Entity A can be assigned to many entities at once, N, and Entity B can also have many Entities assigned to it, M. An example is that a student can have many or no classes and a class can have many or no students in it

Question 22

What is a Strong Entity Type?

Answer 22

A strong entity is complete by itself and is not dependent on any other entity type. It possess a primary key which describes each instance in the strong entity set uniquely

Question 23

What is a Weak Entity Type? Describe an example.

Answer 23

A Weak Entity is an entity that cannot be uniquely identified by its attributes alone; therefore, it must use a foreign key in conjunction with its attributes to create a primary key

An Example is rooms at a hotel. Entity type “room” cannot be uniquely identified using room number, since all hotels have a room #1, so you must use the hotel number pulled from entity “hotel” as an attribute, making “room” a weak entity

Question 24

How is a Weak Entity depicted in an ER model? How are borrowed attributes depicted?

Answer 24

A double-lined rectangle for Weak Entity

Dashed underline for borrowed attributes

Question 25

What is a Ternary Relationship Type?

Answer 25

A Ternary Relationship Type is a relationship type where there are more than two entities engaging in the relationship

Question 26

What is a limitation of the ER model?

Answer 26

• Temporal constraints cannot be enforced (Ex. after two months, … happens)
• Business rules cannot be enforced (Ex. employees can only work on projects undertaken by the department they are in)
• Domains cannot be set or enforced (Ex. hours worked must be a positive value)

Question 27

What is the difference between an ER Model and an EER Model?

Answer 27

An EER Model is just an ER Model containing semantic data modeling concepts, which are specialization/generalization, categorization, and aggregation

Question 28

What are the three extensions to the ER Model that the EER Model provides?

Answer 28

Specialization/Generalization
Categorization
Aggregation

Question 29

What is Specialization?

Answer 29

Specialization refers to the process of defining a set of subclasses of an entity type. These are formed using distinguishing characteristics of entities in a superclass

Think of it like specializing your business degree. You are part of the business student superclass, but then can specialize into finance, marketing, and supply chain

Question 30

What is Generalization?

Answer 30

The reverse of specialization, taking subclasses and grouping them into a superclass

For example, there are finance, marketing, and supply chain students, but they can all be considered as business students

Question 31

What is Disjoint Specialization?

Answer 31

Disjoint Specialization is when an entity can be a member of at most one subclass at a time. A finance student cannot be a supply chain student

Question 32

What is an Overlap Specialization?

Answer 32

Overlap Specialization is when an entity can be a member of more than one subclass at a time. A university student may be both a business and economics major

Question 33

What is Total Specialization?

Answer 33

Total Specialization is when every entity in the superclass must be a member of a subclass. Every university student must have a major

Question 34

What is Partial Specialization?

Answer 34

Partial Specialization allows an entity to be either part of a subclass or not part of a subclass. You can be a university student but not have a minor

Question 35

What are D.O.T.P specializations? Explain each.

Answer 35

Disjoint Specialization
Overlap Specialization
Total Specialization
Partial Specialization

Question 36

What is Categorization? How is it represented in an EER diagram?

Answer 36

Categorization is a subclass that has several possible superclasses. For example, a person (superclass) and a company (superclass) may be of the subclass account holders

Question 37

What is Total Categorization and Partial Categorization?

Answer 37

Total Categorization is when all entities of a superclass belong to a subclass. Partial Categorization is when not all entities of a superclass belong to the subclass

For example, not all companies and people are account holders

Question 38

What is Aggregation? Give an example.

Answer 38

Aggregation is when entity types are related by a particular relationship and are aggregated into a higher-level entity type that has a relationship with other entity types

An example is the entity types “project” and “employee”, which have a “works for” relationship, which can be aggregated and then related to a required machinery