External Level (View Schema)
    ↓ Logical Data Independence
Conceptual Level (Logical Schema)
    ↓ Physical Data Independence  
Internal Level (Physical Schema)

┌─────────────────────────────────────────────────┐
│               ER DIAGRAM SYMBOLS                │
├─────────────────────────────────────────────────┤
│  ┌───────┐                                      │
│  │       │  Rectangle = Entity                  │
│  └───────┘                                      │
│                                                 │
│     ○      Ellipse = Attribute                  │
│                                                 │
│     ◇      Diamond = Relationship               │
│                                                 │
│    ___     Underline = Primary Key              │
│                                                 │
│    (-)     Dashed Ellipse = Derived Attribute   │
│                                                 │
│   {  }     Double Ellipse = Multivalued Attr    │
└─────────────────────────────────────────────────┘

# Entity: STUDENT
# Attributes: student_id (PK), name, email, dob, age (derived)

# Entity: COURSE  
# Attributes: course_id (PK), course_name, credits

# Relationship: ENROLLS
# Student <--(enrolls)--> Course
# Cardinality: Many-to-Many (M:N)
# A student can enroll in many courses
# A course can have many students

# Relationship Attributes:
# enrollment_date, grade

1:1 (One-to-One)
┌───────┐      ┌───────┐
│ Person │──────│ Passport│
└───────┘      └───────┘
One person has exactly one passport

1:N (One-to-Many)
┌───────┐      ┌───────┐
│ Dept  │──────<│ Employee│
└───────┘      └───────┘
One department has many employees

M:N (Many-to-Many)
┌───────┐>─────<┌───────┐
│Student│      │ Course │
└───────┘      └───────┘
Many students enroll in many courses

        ┌─────────┐
        │ VEHICLE │  ← Generalized Entity
        └────┬────┘
             │
    ┌────────┼────────┐
    ↓        ↓        ↓
┌─────┐  ┌─────┐  ┌─────┐
│ Car │  │ Truck│  │ Bike │
└─────┘  └─────┘  └─────┘

        ┌──────────┐
        │ EMPLOYEE │  ← Superclass
        └────┬─────┘
             │ is-a
    ┌────────┼────────┐
    ↓        ↓        ↓
┌───────┐ ┌───────┐ ┌───────┐
│ Manager│ │Engineer│ │ Intern │
└───────┘ └───────┘ └───────┘
            ↑ Subclasses

# Example: Project Management
# Relationship "Works_On" between Employee and Project
# This relationship itself participates in "Manages" relationship

# Employee ---(works_on)--- Project
#               |
#               | (aggregation)
#               ↓
#           [Monitors]
#               ↑
#           Manager

-- STUDENT Table Example
CREATE TABLE Student (
    student_id INT PRIMARY KEY,      -- Primary Key
    email VARCHAR(100) UNIQUE,       -- Candidate Key
    name VARCHAR(50) NOT NULL,
    dept_id INT,
    FOREIGN KEY (dept_id) REFERENCES Department(dept_id)
);

-- Key Types:
-- Super Key: Any set of attributes that uniquely identifies tuples
--   {student_id}, {student_id, name}, {email}, {email, name}

-- Candidate Key: Minimal super key (no redundant attributes)
--   {student_id}, {email}

-- Primary Key: Chosen candidate key for identification
--   student_id

-- Foreign Key: References primary key of another table
--   dept_id references Department(dept_id)

-- Alternate Key: Candidate keys not chosen as primary key
--   email

# Step 1: Map Regular Entities
# Each entity → Table
# Simple attributes → Columns
# Composite attributes → Component attributes as columns
# Multivalued attributes → Separate table with FK

# Step 2: Map Weak Entities
# Include partial key + owner's PK as composite primary key

# Step 3: Map Binary 1:1 Relationships
# Add FK to either side (preferably total participation side)
# Or create separate relationship table

# Step 4: Map Binary 1:N Relationships
# Add FK to "many" side (N-side entity)
# Relationship attributes go with FK

# Step 5: Map Binary M:N Relationships
# Create new relationship table
# Include PKs of both entities as composite PK
# Add relationship attributes to this table

# Step 6: Map Multivalued Attributes
# Separate table with attribute + foreign key to owner

# EXAMPLE:
# Student (M) --- enrolls --- (N) Course
# Creates: Enrollment(student_id, course_id, grade, date)

-- Original ER: Student --enrolls-- Course (M:N)

-- Resulting Tables:
CREATE TABLE Student (
    student_id INT PRIMARY KEY,
    name VARCHAR(50),
    email VARCHAR(100)
);

CREATE TABLE Course (
    course_id INT PRIMARY KEY,
    course_name VARCHAR(100),
    credits INT
);

-- Relationship becomes a table
CREATE TABLE Enrollment (
    student_id INT,
    course_id INT,
    enrollment_date DATE,
    grade CHAR(1),
    PRIMARY KEY (student_id, course_id),
    FOREIGN KEY (student_id) REFERENCES Student(student_id),
    FOREIGN KEY (course_id) REFERENCES Course(course_id)
);