L2: Entity-Relationship Model

Question 1

Benefits of using

Subclasses in

E/R Diagrams

Answer 1

• Certain entity sets might have special cases
• Allows inheriting the entity set’s attributes
  
  • Creates a subclass with new, specialized attributes
• Useful to reduce nulls

Question 2

E/R Diagram:

Types of Constraints

Answer 2

• Key Constraints
• Single Value Constraints
• Participation Constraints
• Referential Constraints
• Numerical Constraints

Question 3

Converting E/R Diagrams

into Relations

(Basic Idea)

Answer 3

• Entities become Relations (tables)
• Relationships ALSO become Relations
• Attributes become the Columns within the Relations
• Relationships use the Keys of the related entities, plus any additional attributes
• Many-to-One:
  
  • The Entity Set with “Many” just has a Foreign Key to the set with “One”

Question 4

E/R Model:

Parts of the Model:

Rectangles

Answer 4

Rectangles represent

Entity Sets

Basically Classes or Types of Entities

Question 5

What is Tuple Variable Ambiguity?

Answer 5

When two different tables (tuples) have the same variable name.