Chapter 4 - Entity Relationship Modelling (ERM)

Question 1

Entities

Answer 1

🐷Refers to entity set and not to single entity occurrence
🐷Corresponds to table and not to row in relational environment
🐷In Chen and Crow’s Foot models, entity is represented by rectangle with entity’s name
🐷The entity name, a noun, is written in capital letters

Question 2

Attributes

Answer 2

• Characteristics of entities
• Chen notation: Ovals containing its names connected to entity rectangle with a line.
  Crow’s foot notation: attributes written in attribute box below entity rectangle

Question 3

Required attribute

Answer 3

Must have a value

Question 4

Optional attribute

Answer 4

may be left empty

Question 5

Domain

Answer 5

Set of possible values for an attribute

- Attributes may share the same domain

Question 6

Identifiers

Answer 6

One or more attributes that uniquely identify each entity instance

Question 7

Composite identifier

Answer 7

Primary key composed of more than one attribute

Question 8

Which attribute can be subdivided?

Answer 8

Composite attribute

Question 9

Which attribute cannot be subdivided?

Answer 9

Simple attribute

Question 10

Single-value attribute

Answer 10

can have only a single value

Question 11

Multivalued attribute

Answer 11

can have many values

Question 12

Derived attribute

Answer 12

• value may be calculated from other attributes

- need not to physically stored within database

Question 13

Participants

Answer 13

are entities that participate in a relationship

Question 14

Connectivity

Answer 14

describe the relationship classification

Question 15

Cardinality

Answer 15

expresses minimum and maximum number of entity occurrences associated with one occurrence of related entity

Question 16

Existence dependence

Answer 16

Entity exists in database only when it is associated with another related entity occurrence

Question 17

Existence independence

Answer 17

can exist apart from one or more related entities

-referred to as a strong or regular entity

Question 18

Weak (non-identifying) relationships

Answer 18

Exists if PK of related entity does not contain PK component of parent entity

Question 19

Strong (identifying) relationships

Answer 19

Exists when PK of related entity contains PK component of parent entity

Question 20

Weak Entities

Answer 20

• Existence-dependent

- Primary key partially or totally derived from parent entity in relationship

Question 21

Optional participation

Answer 21

– One entity occurrence does not require corresponding entity occurrence in particular relationship

Question 22

Mandatory participation

Answer 22

– One entity occurrence requires corresponding entity occurrence in particular relationship

Question 23

Relationship Degree

Answer 23

the number of entities or participants associated with a relationship

Question 24

Unary relationship

Answer 24

Association is maintained within single entity

Question 25

Binary relationship

Answer 25

Two entities are associated

Question 26

Ternary relationship

Answer 26

Three entities are associated

Question 27

Recursive Relationships

Answer 27

Relationship can exist between occurrences of the same entity set
–Naturally found within unary relationship

Question 28

Associative (Composite) Entities

Answer 28

• Also known as bridge entities
• Used to implement M:N relationships
• Composed of primary keys of each of the entities to be connected
• May also contain additional attributes that play no role in connective process

Question 29

relational schema

Answer 29

table structure

Question 30

iterative process

Answer 30

A process based on repetition of steps and procedures.

Question 31

Building an ERD usually involves the following activities:

Answer 31

• Create detailed narrative of organization’s description of operations
• Identify business rules based on description of operations
• Identify main entities and relationships from business rules
• Develop initial ERD
• Identify attributes and primary keys that adequately describe entities
• Revise and review ERD