Chapter 4

Question 1

ERM forms…

Answer 1

The basis of an entity relationship diagram (ERD)
-Conceptual database as viewed by the end user.

Question 2

Database main components

Answer 2

-Entities
-Attributes
-Relationships

Question 3

Entities

Answer 3

Object of interest to the end user
-Refers to the entity set and not to a single entity occurrence.

Question 4

ERM corresponds to…

Answer 4

A table in the relational environment

Question 5

Entity in Chen, Crow’s foot and UML

Answer 5

An entity is represented by a rectangle that contains the entity’s name.
-usually written in all capital letters.

Question 6

Characteristics of Entities

Answer 6

-Required attribute
-Optional attribute
-Domain
-Identifier (PK)

Question 7

Required attribute

Answer 7

An entity must have a value and cannot be empty.

Question 8

Optional attribute

Answer 8

Does not require a value and can be left empty.

Question 9

Domain

Answer 9

A set of possible values for a given attribute.

Question 10

Identifier (PK)

Answer 10

One or more attributes that uniquely identify each entity instance.

Question 11

More characteristics of entities

Answer 11

-Composite identifier
-Composite attribute
-Simple attribute
-Single-valued attribute
-Multivalued attributes

Question 12

Composite identifier

Answer 12

Primary key composed of more than one attribute

Question 13

Composite attribute

Answer 13

Attribute that can be subdivided to yield additional attributes.

Question 14

Simple attribute

Answer 14

Attribute that cannot be subdivided.

Question 15

Single-valued attribute

Answer 15

Attribute that has a single value.

Question 16

Multivalued attributes

Answer 16

Attributes that have many values.

Question 17

Requirements of multivalued attributes

Answer 17

-Create several new attributes, one for each component of the original multivalued attribute
-Develop a new entity composed of the original multivalued attribute’s components.

Question 18

Derived attribute

Answer 18

Attribute whose value is calculated from other attributes (derived using an algorithm).

Question 19

Advantage of storing a derived attribute

Answer 19

-Saves CPU processing cycles
-Saves data access time
-Data value is readily available
-Can be used to keep track of historical data

Question 20

Disadvantages of storing derived attributes

Answer 20

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

Question 21

Advantage of not storing derived attributes

Answer 21

-Saves storage space
-Computation always yields current value

Question 22

Disadvantage of not storing derived attribute

Answer 22

-Uses CPU processing cycles
-Increases data access time
-Adds coding complexity to queries.

Question 23

Participants

Answer 23

Entities that participate in a relationship

Question 24

Connectivity

Answer 24

Describes relationship classification (1:1, 1:M, etc)

Question 25

Cardinality

Answer 25

Expresses the minimum and maximum number of entity occurrences associated with one occurrence of related entity.

Question 26

Existence dependence

Answer 26

An entity exists in the database only when it is associated with another related entity occurrence.

Question 27

Existence independence

Answer 27

-Entity exists apart from all of its related entities -Referred to as a strong entity or a regular entity.

Question 28

Weak (non-identifying) relationship

Answer 28

Primary key of the related entity does not contain a primary key component of the parent entity

Question 29

Strong (identifying) relationships

Answer 29

Primary key of the related entity contains a primary key component of the parent entity.

Question 30

Conditions of a weak entity

Answer 30

-Existence-dependent -Has a primary key that is partially or totally derived from parent entity in the relationship

Question 31

Optional participation

Answer 31

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

Question 32

Mandatory participation

Answer 32

One entity occurrence requires a corresponding entity occurrence in a particular relationship.

Question 33

Unary relationship

Answer 33

Association is maintained within a single entity

Question 34

Recursive relationships

Answer 34

Relationship exists within a single entity type

Question 35

Binary relationship

Answer 35

Two entities are associated

Question 36

Ternary relationship

Answer 36

three (or more) entities are associated

Question 37

Relationship can exist...

Answer 37

Between occurrences of the same entity set -Naturally, such a condition is found within a unary relationship

Question 38

Pitfall of unary relationships

Answer 38

One common pitfall when working with unary relationships is to confuse participation with referential integrity -Similar because they are both implemented through constraints on the same set of attributes.

Question 39

Composite (Associative) Entities

Answer 39

Used to represent an M:N relationship between two or more entities

Question 40

Activities involved in building an ERD

Answer 40

- Create a detailed narrative of the organisation's description of operations. -Identify business rules based on the descriptions -Identify main entities and relationships from the business rules -Develop the initial ERD -Identify the attributes and primary keys that adequately describe entities -Revise and review ERD