Chapter 3 Entity-Relationship (ER) Model

Question 1

What is an entity and what is an attribute?

Answer 1

An entity is an object or concept in the database, such as a “customer.” An attribute is a characteristic of an entity, such as “name” or “address.”

Question 2

What is the difference between simple and composite attributes?

Answer 2

A simple attribute cannot be divided further (e.g., a person’s first name). A composite attribute can be divided into smaller subparts (e.g., an address can be divided into street, city, and zip code).

Question 3

What is the difference between single-valued and multi-valued attributes?

Answer 3

A single-valued attribute holds only one value (e.g., a person’s birthdate), while a multi-valued attribute can hold multiple values (e.g., a person’s phone numbers).

Question 4

What is the difference between stored and derived attributes?

Answer 4

A stored attribute holds data that is physically stored in the database, while a derived attribute is calculated from other attributes (e.g., age derived from birthdate).

Question 5

What are cardinality, degree, and optionality in relationships?

Answer 5

Cardinality refers to the number of instances of an entity involved in the relationship. Degree is the number of entities involved in the relationship. Optionality refers to whether an entity’s participation in a relationship is mandatory or optional.

Question 6

What is a weak entity?

Answer 6

A weak entity is an entity that cannot be uniquely identified by its own attributes alone and relies on a strong (parent) entity for identification.

Question 7

What are supertype and subtype entities?

Answer 7

A supertype is a general entity that contains common attributes for related subtypes, which represent more specialized entities.