CHP6

Question 1

Levels of data modelling:

Answer 1

1. Conceptual (Communication and definition)
2. Logical (Clarification and detail)
   3, Physical (Technical implementation)

Question 2

Main components of an ERD (entity relational diagram)

Answer 2

1. Attributes
2. Entities
3. Relationships

Question 3

Domains

Answer 3

• Attributes have domains and may share them
• For instance, gender has two possibilities male or female
• Students and professor may share the same address

Question 4

Identifiers

Answer 4

• They are underlined in the ERD
• Primary Key
• EG CAR(CAR_YEAR,CAR_COLOUR)
• The car is the primary key and inside the parameters are its attributes

Question 5

Composite and simple attributes

Answer 5

• Composite attributes can be subdivided
• For instance, Address can be subdivided into street, city, state
• Simple attributes cannot be subdivided for instance sex, age

Question 6

Composite key

Answer 6

• In certain circumstances, primary key can be used more than once

Question 7

Derived attributes

Answer 7

• Only exist if a value from an attribute is calculated from another attribute.
• Doesn’t have to be physically present inside the database
  ADVTG: Saves storage space
  DSADVTG: Requires constant maintenance to keep it up-to-date

Question 8

Strong and Weak entity

Answer 8

WEAK - is dependent on another entity and cannot exist otherwise
- has a primary key that is partially derived from a parent entity (PK & FK)
STRONG - is independent