ER Modeling

Question 1

Miniworld

Answer 1

part of the real world that we are interested in modeling

Question 2

What is the importance of conceptual design?

Answer 2

• Fundamental
• Involves users
• Provides good documentation for the Db
• Independent of the DBMS

Question 3

What is the Entity-Relationship (ER) model?

Answer 3

Notion that a database can be modelled as a collection of entity classes and relationships among them

Question 4

What is the goal of the ER model?

Answer 4

Develop an ER schema or ER diagram

Question 5

Entity

Answer 5

Data object that exists and is distinguishable from other objects

Question 6

What can be considered entities?

Answer 6

An object that physically exists, abstract or organizational entity, or an event

Question 7

Entity type

Answer 7

description of the attributes that a set of entities has in common

Question 8

Entity set

Answer 8

A collection of of entities of the same type

Question 9

Entity class

Answer 9

combination of entity set + entity type

Question 10

Attribute

Answer 10

Property of an entity

Question 11

All members of an entity class will have the same ______

Answer 11

Attributes
But they may have different attribute values
(e.g. all customers will have a name but the name will likely be different)

Question 12

How is an entity class represented?

Answer 12

A rectangle

Question 13

How is an attribute represented?

Answer 13

labelled oval connected to a entity class

Question 14

How are entities distinguished from one another?

Answer 14

Each entity will have a unique combination of attribute values

Question 15

Candidate Identifier

Answer 15

a set of one or more attributes whose values uniquely determine each entity.

Question 16

Can you have more than one candidate identifier?

Answer 16

Yes there can be several

Ex: Name, DOB of customers, customerid of customers

Question 17

Can you have more than one primary identifier?

Answer 17

No. One candidate identifier is selected to be the primary identifier

Question 18

What conditions must be met for a primary identifier?

Answer 18

• Unique
• Mandatory
• Time invariant

Question 19

How are primary identifier represented?

Answer 19

Labelled oval with an underline

Question 20

How are null values represented? When are they used?

Answer 20

@ symbol

For unknown or inapplicable values

Question 21

How are multi-valued attributes represented? What is an example?

Answer 21

Double oval

Customer could have more than 1 phone number

Question 22

What is a composite attribute? What is the opposite of a composite attribute?

Answer 22

A composite attribute is comprised of multiple values to describe it (e.g. address has street, city, zip etc.)
The opposite of a composite is a simple

Question 23

What is a derived attribute? How is it represented? What is the opposite of a derived attribute?

Answer 23

The opposite of derived is stored. It is an attribute that is calculated from another attribute. It’s represented with a dotted oval. An example being age calculated from date of birth attribute

Question 24

Where are characteristics captured that are not typically represented in the ER diagram? What are some examples?

Answer 24

Data dictionary

• Attribute cardinality
• Entity class cardinality
• Attributes that are optional or mandatory, attribute uniqueness
• Domain constraints (e.g SSN must be 9 exactly)

Question 25

Cardinality

Answer 25

Number of elements in that set

Question 26

Cardinality constraint

Answer 26

bounds the upper and/or lower limits of the cardinality

Question 27

What does a cardinality constraint do for an attribute?

Answer 27

Restricts the number of values an attribute can take on

Question 28

Interaction relationship

Answer 28

association between or among entities

Question 29

Relationship type

Answer 29

is a list (ordered) of participating entity classes

Question 30

Relationship set

Answer 30

specific set of associated entity instances based on relationship type

Question 31

Relationship Class

Answer 31

relationship type + relationship set

Question 32

How is a relationship class represented?

Answer 32

Diamond

Question 33

Can a relationship class have attributes?

Answer 33

No

Question 34

How is data that exists due to the relationship classified as?

Answer 34

Weak entity | The data has no meaningful existence without the relationship that defines it (e.g order details)

Question 35

What is the purpose of a weak entity?

Answer 35

Provide more data about the interaction between the 2 entities

Question 36

Degree of an interaction relationship

Answer 36

Number of entity classes involved in a relationship

Question 37

What are examples of interaction relationships?

Answer 37

unary binary ternary

Question 38

Business Rule

Answer 38

business policy that leads to a rule/statement constraining some aspect of the business

Question 39

Where are Business rules and other or behavior-oriented/process oriented rules located?

Answer 39

Annotations on the ER or data dictionary

Question 40

Weak entity class

Answer 40

inherits identifying attributes from another entity class to uniquely identify its own entities

Question 41

How is a weak entity class represented?

Answer 41

Double lined box and an arrow pointing to the identifying relationship

Question 42

What is the defining characteristic of a weak entity?

Answer 42

If an entity class depends on another class for all or part of its identifier, it's weak

Question 43

When does a weak entity have a partial identifier?

Answer 43

If the max cardinality > 1 in its identifying relationship

Question 44

How is a partial identifier represented?

Answer 44

Dotted underline

Question 45

What is the circle relationship symbol?

Answer 45

Indicates a superclass and associated subclass

Question 46

Designation

Answer 46

Classifying attribute for the superclass/subclass relationship

Question 47

What are the 4 types of superclass/subclass relationships

Answer 47

Disjoint, 0:1 Partition, 1:1 Overlap, 0:M Cover, 1: M

Question 48

Disjoint superclass/subclass relationship

Answer 48

0:1 relationship Example: an employee may not be an associate or a manager (just in the general employee pool)

Question 49

Partition

Answer 49

1:1 Example: Employee must either be a manager or an associate. No employee can be both

Question 50

Overlap

Answer 50

0:M | An employee can be both a manager and an engineer (or could be neither)

Question 51

Cover

Answer 51

1:M | Employee has to be either a manager or engineer or could be both (can't be neither)

Question 52

When to create subclasses

Answer 52

- Attributes relative only to a subclass - Relationship classes relevant only to a subclass - Cardinality of relationships associated with subclasses are different

Question 53

How do subclasses gain attributes and relationships?

Answer 53

They are inherited from the superclass (including the identifier)

Question 54

Specific attributes

Answer 54

attributes to the subclass only

Question 55

Specific relationships

Answer 55

entities in a subclass participating in direct relationships