Unit 6 : Entity Relationship Model

Question 1

What ERD represents to end user?

Answer 1

1. Represents conceptual database as viewed by end user

Question 2

List out the main components of ERD’s depict database ( 3 )

Answer 2

1. Entities
2. Attributes
3. Relationships

Question 3

What does ER model forms?

Answer 3

1. The basis of an ERD diagram

Question 4

What does entities refer to?

Answer 4

1. Refers to entity set and not to single entity occurence

• Table in relational environment

Question 5

What models can we draw?

Answer 5

1. Chen Models
2. Crow’s Foot Models

• Chen Models are like a fan , whle Crow’s is like a foot

Question 6

Do attribute have domain?

Answer 6

1. Yes, it may also share a domain

• Domain is attribute’s set of possible values

Question 7

How can we know there is primary keys in ERD?

Answer 7

1. Underlined

Question 8

What is a composite primary keys ( Composite Keys )

Answer 8

1. Primary keys ideally comprosed of more than one attribute

Question 9

Is it possible to use composite key for more than one attribute

Answer 9

1. Yes

Question 10

What can be subdivided? What can’t be subdivided? ( Composite Attribute, Simple Attribute )

Answer 10

1. Composite Attribute
2. Simple Attribute

Question 11

A single-value attribute can have how many attributes?

Answer 11

1. Single Value

Question 12

A multivalued attribute can have how many attributes?

Answer 12

1. Many Value

Question 13

What are derived attributes?

Answer 13

1. Attribute whose value may be calculated ( derived ) from other attributes

Question 14

Does derived attributes needed to be physically stored within database?

Answer 14

1. No

• It can be derived by using an algorithm

Question 15

What are the advantages on stored derived attribute? ( 3 )

Answer 15

1. Saves CPU processing cycles
2. Data value is readily available
3. Can be used to track of historical data

Question 16

What is the disadvantages of stored derived attribute?

Answer 16

1. Requires constant maintenance to ensure derived value is current, especially if any values used in the calculation change

Question 17

What are the advantages in not stored derived attribute? ( 2 )

Answer 17

1. Saves storage space
2. Computation always yields current value

Question 18

What are the disadvantages in not stored derived attribute

Answer 18

1. Uses CPU processing cycles
2. Adds coding complexity to queries

Question 19

What relationships than can be classified as?

Answer 19

1. 1 : M

Question 20

Will relationships between entities always operate in both directions?

Answer 20

1. Yes

• Always operate in both directions

Question 21

What is the term connectivity used?

Answer 21

1. Used to describe the realtionship classification

Question 22

What is the term cardinality used?

Answer 22

1. Expresses minimum and maximum number of entity occurences associated with one occurence of related entity

Question 23

What can connectivity and cardinality established?

Answer 23

1. established by very concise statements known as business rules

Question 24

Who usually determines an entity can be described as weak based on what ?

Answer 24

1. Database designer
2. Business Rules