Chapter 5

Question 1

Entity supertype

Answer 1

Generic entity type that contains the common characteristics of entity subtypes

Question 2

Entity subtype

Answer 2

• a subset of an entity supertype

- Contains unique characteristics of each entity

Question 3

Disjoint subtypes

Answer 3

these are unique and nonoverlapping subtype entity set

Question 4

Overlapping subtypes

Answer 4

• contain nonunique subsets of the supertype entity set

- a condition in which each entity instance(row) of the supertype can appear in more than one subtype

Question 5

Partial completeness

Answer 5

not every supertype occurrence is a member of a subtype

Question 6

Total completeness

Answer 6

every supertype occurrence must be a member of at least one subtypes

Question 7

entity cluster (Virtual entity type)

Answer 7

• used to represent multiple entities and relationships in ERD
• Formed by combining multiple interrelated entities into a single, abstract entity object
• General rule: avoid the display of attributes to eliminate complications that result when the inheritance rules change

Question 8

Primary key

Answer 8

• single attribute or a combination of attributes
• Uniquely identifies each entity instance
• Guarantees entity integrity
• Works with foreign keys to implement relationships

Question 9

Natural key or natural identifier

Answer 9

• real-world identifier used to uniquely identify real-world objects
• Familiar to end users and forms part of their day-to-day business vocabulary
• Used as the primary key of the entity being modeled

Question 10

Desirable primary key characteristics

Answer 10

• Non intelligent
• No change over time
• Preferably single-attribute
• Preferably numeric
• Security-compliant

Question 11

Identifiers of composite entities

Answer 11

Each primary key combination is allowed once in M:N relationship

Question 12

Identifiers of weak entities

Answer 12

• Strong identifying relationship with the parent entity
• Represents a real-world object that is existence-dependent on another real-world object
• Represented in the data model as two separate entities in a strong identifying relationship

Question 13

When to use surrogate primary keys

Answer 13

• Useful when there is no natural key

- Helpful if selected candidate key has embedded semantic contents or is too long

Question 14

Implementing 1:1 Relationships (1 of 3)

Place primary key

Answer 14

Place primary key of the parent entity on the dependent entity as foreign key

Question 15

Implementing 1:1 Relationships (1 of 3)

Options for selecting and placing the foreign key

Answer 15

• Place a foreign key in both entities

- Place a foreign key in one of the entities

Question 16

Time-variant data

Answer 16

data whose values change over time and for which a history of the data changes must be retained

Question 17

Time-variant data requires

Answer 17

creating a new entity in a 1:M relationship with the original entity

Question 18

Design trap

Answer 18

• Occurs when a relationship is improperly or incompletely identified
• Data represented in a way not consistent with the real world

Question 19

Fan trap

Answer 19

• Occurs when one entity is in two 1:M relationships to other entities
• Produces an association among other entities not expressed in the model

Question 20

Inheritance, in the EERD

Answer 20

the property that enables an entity subtype to inherit the attributes and relationships of the supertype

Question 21

subtype descriminator

Answer 21

the attribute in the supertype that determines to which entity subtype each supertype occurrence is related

Question 22

specialization

Answer 22

the grouping of unique attributes into a subtpye entity

Question 23

generalization

Answer 23

the grouping of common attributes into a supertype entity

Question 24

surrogate key

Answer 24

system-assigned primary key, generally numeric and auto-incremented