Database Concepts: Supertype and Subtype

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Questions and Answers

What is the primary purpose of dividing an entity set into subtypes?

To add unique attributes specific to subclasses (correct)

To simplify relationship management

To merge similar entities into one

To eliminate redundancy in attributes

In the hierarchy of Employees, what type of relationship exists between Employees and Contract_Emps?

One-to-many

Aggregation relationship

ISA relationship (correct)

Many-to-many

Which of the following attributes would logically exist in a subclass, but not in the superclass Employees?

dname

hourly_wage (correct)

salary

ssn

When identifying relationships between Employee and Department, which statement is true?

Each department is managed by a single employee. Signup and view all the answers

What is a characteristic of a subclass in an entity hierarchy?

It inherits all attributes from the superclass. Signup and view all the answers

In the given database structure, which entity set requires a unique identifier based on another entity?

Children of Employees Signup and view all the answers

What is NOT a reason for identifying subclasses in an entity-relationship model?

To simplify data retrieval processes Signup and view all the answers

What relationship can be inferred from the fact that employees work in departments?

Each department is associated with multiple employees. Signup and view all the answers

What is the primary characteristic of an entity type?

It represents a collection of entities with shared properties. Signup and view all the answers

Which of the following represents an entity instance in the context of a university database?

A specific student named John Doe. Signup and view all the answers

In the provided scenario, which of the following is not considered an independent entity?

TITLE Signup and view all the answers

What best describes the entity set CUSTOMER in the bookshop scenario?

The collection of all distinct individuals who purchase books. Signup and view all the answers

Which component of an ER diagram is specifically used to represent a relationship?

Diamond Signup and view all the answers

What defines the attribute of an entity in ER modeling?

It represents specific properties or characteristics of the entity. Signup and view all the answers

Which entity set includes all addresses in the system?

ADDRESS Signup and view all the answers

In conceptual data modeling, what is the significance of filtering potential entities?

To eliminate unnecessary entities to refine the model. Signup and view all the answers

What is a characteristic of a one-to-one (1:1) relationship?

An instance of one entity is associated with exactly one instance of another entity. Signup and view all the answers

How is a many-to-many (M:M) relationship represented in a relational model?

Through the use of an associative entity or bridge table. Signup and view all the answers

Which example correctly illustrates a one-to-many (1:M) relationship?

A user can place multiple orders. Signup and view all the answers

What is an associative entity in the context of ER modeling?

An entity that helps to convert many-to-many relationships into two one-to-many relationships. Signup and view all the answers

What does the 'cardinality' of a relationship refer to in ER diagrams?

The number of instances of one entity related to instances of another entity. Signup and view all the answers

In which scenario is a many-to-many (M:M) relationship applicable?

One employee can opt for multiple policies. Signup and view all the answers

What is the role of the unique identifiers in resolving many-to-many relationships?

They form a new bridge table by linking together the two original entities. Signup and view all the answers

What statement accurately reflects a characteristic of a one-to-many relationship?

One entity can have many relationships with instances of another entity. Signup and view all the answers

Study Notes

Supertype / Subtype

When a group of instances within an entity set share specific properties, it's beneficial to subdivide the entity set into subsets.
Example: Employees can be classified into Contract_Emps and Hourly_Emps.
Contract_Emps and Hourly_Emps inherit attributes from Employees.
Hourly_Emps is a subtype (ISA) of Employees.

Class / ISA Hierarchies

A Supertype is a parent, superclass, or superset entity.
Subtypes are child, subclass, or subset entities.
Subtypes inherit all attributes and relationships from the Supertype.

Reasons for Subclasses

To add attributes that are relevant to specific subtypes, but not the general Supertype (e.g., hourly_wage is specific to Houry_Emps).
To identify specific entities that participate in a relationship (e.g., only Senior_Emps can be managers).

Entity and Entity Type

Entity Type: A collection of entities with shared properties.
Entity Instance: A single, unique occurrence of an Entity Type.
Example: "Student" is an Entity Type; "John Doe" is an Entity Instance of Student.

Cardinality of a Relationship

One-to-one (1:1): One instance of an entity is related to exactly one instance of another entity.
One-to-many (1:M): One instance of an entity is related to multiple instances of another entity.
Many-to-many (M:M): Multiple instances of an entity are related to multiple instances of another entity.

Transforming Many-to-Many Relationships

Many-to-Many relationships are modeled using a bridge table (associative entity).
This bridge table has its own unique identifier often composed of identifiers from the original entities.
It breaks the M:M relationship into two 1:M relationships.
Example: Student and Course (M:M) becomes Student and Enrollment (1:M) and Course and Enrollment (1:M).

Associative Entity

Breaks down many-to-many relationships into two one-to-many relationships.
Often has its own unique attributes.
Example: In a university database, a bridge entity called "Enrollment" can be created to connect students and courses.

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Description

This quiz covers essential database concepts including supertype and subtype relationships, ISA hierarchies, and the purpose of subclasses. Understand how entities inherit properties and why it's important to classify data efficiently.