Chapter 6

Question 1

What does it mean to say that constructing an ERD is an iterative process?

Answer 1

It means that ERD models could be refined as many times as needed

Question 2

Why decompose a compound attribute into smaller attributes?

Answer 2

Because it makes the search for embedded data easier

Question 3

When is it appropriate to expand an attribute?

Answer 3

When additional details about an attribute are needed

Question 4

Why transform an entity type into two entity types and a relationship?

Answer 4

Add a finer level of detail about an entity, usually a two-level structure

Question 5

Why transform a weak entity type to a stronger entity type?

Answer 5

Remove combined foreign keys after conversion to tables

Question 6

Why transform an entity type into a generalization hierarchy?

Answer 6

When there are multiple attributes that do not apply to all entities and when there is a classification of entities

Question 7

Why add a history to an attribute or relationship?

Answer 7

For detailed legal requirements and strategic reporting

Question 8

What changes to an ERD are necessary when expanding an attribute?

Answer 8

The changes require the replacement of an attribute with an entity type and a 1-M relationship.

Question 9

What changes when splitting a compound attribute?

Answer 9

An attribute is replaced with a collection of attributes

Question 10

What changes when expanding an entity type?

Answer 10

An entity type is split into two entity types and a relationship

Question 11

What changes when transforming a weak entity type to a strong entity type?

Answer 11

A weak entity type changes into a regular entity type and converts associated identifying relationship into regular (non-identifying) relationships

Question 12

What changes when adding unlimited history to an attribute or a relationship? what changes when replacing an entity type with a generalization hierarchy?

Answer 12

For attribute history, an attribute is replaced with a weak entity type and an identifying 1-M relationship. For 1-M relationship, the relationship will be changed to an associative entity type along with identifying relationships
Starting from a supertype; add subtypes, a generalization hierarchy, and redistribute attributes to subtypes. Starting from subtypes: add a supertype, a generalization hierarchy and redistribute common attributes and relationships to the supertype.

Question 13

What is overuse of a generalization hierarchy and how is it resolved?

Answer 13

Generalization hierarchies are uncommon but a typical novice uses them inappropriately. It is resolved by ensuring that subtypes have specialized attributes and relationships. Generalization indicates the essence of an entity whereas attributes indicate the functions performed by an entity or state of an entity.

Question 14

How is an M-N relationship converted to a table design?

Answer 14

Each M-N relationship becomes a separate table. The primary key of the table is a combined key consisting of the primary keys of the entity types participating in the M-N relationship.

Question 15

What about a 1-M relationship?

Answer 15

Each 1-M relationship becomes a foreign key in the table corresponding to the child entity type (the entity type near the Crow’s Foot symbol). If the minimum cardinality on the parent side of the relationship is one, the foreign key cannot accept null values.

Question 16

Weak entity type converted to a table design?

Answer 16

Each entity type (except a subtype) becomes a table. The attributes os the entity type becomes columns in the table

Question 17

Generalization hierarchy converted to a table design?

Answer 17

Each entity type of a generalizations hierarchy becomes a table. The columns of a table are the attributes of the corresponding entity typer plus the primary key of the parent entity type. For each table representing a subtype, define a foreign key constraint that references the table corresponding to the parent entity type.

Question 18

A 1-1 relationship to a table design?

Answer 18

Each 1-1 relationship is converted into two foreign keys. If the relationship is optional with respect to one of the entity types, the corresponding foreign key may be dropped to eliminate null values.

Question 19

Criteria for choosing a primary key?

Answer 19

The primary key should be stable and single purpose. Stable means it should not change after it has been assigned to an entity. Single purpose means that the primary key should have no purpose other than entity identification.

Question 20

What to do when a proposed primary key does not meet criteria?

Answer 20

If the proposed primary key does not met the requirement then you need to reject it as the primary key

Question 21

What changes to an ERD are necessary when replacing a collection of entity types with a generalization hierarchy?

Answer 21

The transformation involves the addition of a supertype and a generalization hierarchy. Same as a single entity

Question 22

How is a hub entity type used to simplify an ERD?

Answer 22

An entity type hub has direct relationships with other entity types. Therefore other relationships in an ERD can be eliminated if an indirect relationship exists through a hub entity type.

Question 23

A tuple is synonymous with a field

Answer 23

False