Entity, Attributes, and Relationships in DBMS

Detailed explanation with examples and Oracle SQL for database design.

1. Entity

An entity is a real-world object or concept with identifiable data.

Examples: Student, Course, Teacher

Types of Entities

Strong Entity: Exists independently, has primary key.
Example: Student(student_id, name, dob, course)
Weak Entity: Depends on another entity, no primary key of its own.
Example: Dependent(dep_id, name, emp_id)

2. Attributes

Attributes are properties or characteristics of an entity.

Attribute Type	Example	Notes
Simple	name	Cannot be divided
Composite	FullName → FirstName, LastName	Can split into sub-parts
Derived	age (from dob)	Calculated from other attribute
Multi-valued	phone_numbers	Can have multiple values
Key Attribute	student_id	Uniquely identifies entity

Oracle Table Example

CREATE TABLE Student (
    student_id NUMBER PRIMARY KEY,
    first_name VARCHAR2(50),
    last_name VARCHAR2(50),
    dob DATE,
    course VARCHAR2(50)
);

3. Relationships

A relationship represents how entities are associated.

Type	Example	Description
One-to-One (1:1)	Employee – CompanyCar	Each employee has one car
One-to-Many (1:N)	Teacher – Students	One teacher teaches many students
Many-to-One (N:1)	Students – Department	Many students belong to one department
Many-to-Many (M:N)	Students – Courses	Students enroll in multiple courses; each course has many students

Oracle Example for Many-to-Many Relationship

-- Student Table
CREATE TABLE Student (
    student_id NUMBER PRIMARY KEY,
    name VARCHAR2(50)
);

-- Course Table
CREATE TABLE Course (
    course_id NUMBER PRIMARY KEY,
    course_name VARCHAR2(50)
);

-- Enrollment Table (Relationship)
CREATE TABLE Enrollment (
    student_id NUMBER,
    course_id NUMBER,
    PRIMARY KEY(student_id, course_id),
    FOREIGN KEY(student_id) REFERENCES Student(student_id),
    FOREIGN KEY(course_id) REFERENCES Course(course_id)
);

4. Summary

Entity: Student, Teacher, Course (real-world object)
Attribute: Properties like student_id, name, dob
Relationship: How entities are connected (1:1, 1:N, N:1, M:N)

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🏷️ Entity, Attributes, and Relationships in DBMS with Oracle Examples