Chapter 5

Question 1

Extended entity relationship model (EERM)

Answer 1

-Result of adding more semantic constructs to the original entity relationship model
-Extended entity relationship diagram (EERDs) use the EERM

Question 2

Entity supertype

Answer 2

-Generic entity type that is related to one or more entity subtypes.
-Contains common characteristics.

Question 3

Entity subtype

Answer 3

-Contains unique characteristics of each entity subtype

Question 4

Criteria for supertype and subtype usage

Answer 4

-There must be different, identifiable kinds of the entity in the user’s environment.
-The different kinds of instances should each have one or more attributes that are unique to that kind of instance

Question 5

Specialisation Hierarchy

Answer 5

-Depicts parent and child entities

Question 6

Specialisation hierarchy details

Answer 6

-Depicts arrangement of higher-level entity supertypes and lower-level entity subtypes
-Relationships are described in terms of “is-a” relationships
-Subtype exists within the context of a supertype
-Every subtype has one supertype to which it is directly related
-Supertype can have many subtypes.

Question 7

A specialisation hierarchy provides means to…

Answer 7

-Support attribute inheritance
-Define a special supertype attribute known as the subtype discriminator
-Define disjoint or overlapping constraints and complete or partial constraints

Question 8

Inheritance

Answer 8

Enables an entity subtype to inherit attributes and relationships of the supertype

Question 9

Inheritance details

Answer 9

-All entity subtypes inherit their primary key attribute from their supertype
-At the implementation level, supertype and its subtypes maintain a 1:1 relationship
-Entity subtypes inherit all relationships in which supertype entity participates
-Lower-level subtypes inherit all attributes and relationships from its upper-level supertypes.

Question 10

Subtype discriminator

Answer 10

Attribute in the supertype entity that determines to which entity subtype the supertype occurrence is related
-Default comparison condition is the equality comparison
-In some situations, the subtype discriminator is not necessarily based in an equality comparison

Question 11

Disjoint subtypes

Answer 11

Contain a unique subset of the supertype entity set (known as nonoverlapping subtypes)

Question 12

Overlapping subtypes

Answer 12

Contain nonunique subsets of the supertype entity set (implementation requires the use of one discriminator attribute for each type)

Question 13

Completeness constraint

Answer 13

Specifies whether each supertype occurrence must also be a member of at least one subtype

Question 14

Types of completeness constraints

Answer 14

-Partial completeness
-Total completeness

Question 15

Partial completeness

Answer 15

Not every supertype occurrence is a member of a subtype

Question 16

Total completeness

Answer 16

Every supertype occurrence must be a member of at least one subtype

Question 17

Entity clustering

Answer 17

Virtual entity type used to represent multiple entities and relationships in ERD
-Formed by combining multiple interrelated entities into a single, abstract entity object

Question 18

Entity clustering general rule

Answer 18

Avoid the display of attributes to eliminate complications that result when the inheritance rules change

Question 19

Entity integrity: Primary keys

Answer 19

Single attribute or combination of attributes
-Uniquely identifies each entity instance
-Works with foreign keys to implement relationships

Question 20

Natural key or natural identifier

Answer 20

Real-world identifier used to uniquely identify real-world objects
-Stuff like invoice number or student number

Question 21

Desirable primary key characteristics

Answer 21

-Non intelligent
-No change over time
-Preferably single-attribute
-Preferable numeric
-Security-complaint

Question 22

Identifiers of composite entities

Answer 22

-Each primary key combination is allowed once in M:N relationships

Question 23

Identifiers of weak entities

Answer 23

Strong identifying relationship with the parent entity

Question 24

Identifiers of weak entities

Answer 24

-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 25

When to use surrogate primary keys

Answer 25

Primary key used to simplify the identification of entity instances are useful when: -There is no natural key -The selected candidate key has embedded semantic contents or is too long -Requires ensuring that the candidate key of entity in question performs properly

Question 26

Foreign keys...

Answer 26

work with primary keys to properly implement relationships in relational model -Rule: place primary key of the parent entity on the dependent entity as foreign key

Question 27

Options for selecting and placing the foreign key

Answer 27

-Place a foreign key in both entities -Place a foreign key in one of the entities

Question 28

Time-variant data

Answer 28

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

Question 29

Design trap

Answer 29

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

Question 30

Fan trap

Answer 30

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