Aspect	Primary Key	Foreign Key
Definition	Uniquely identifies each record in a table.	Creates a relationship between two tables.
Uniqueness	Must be unique for each record.	Can have duplicate values.
NULL Values	NULLs are NOT allowed.	NULLs are allowed (if no relation exists).
Number per Table	Only one primary key per table.	A table can have multiple foreign keys.
Purpose	Ensures that each record is unique and identifiable.	Ensures referential integrity between tables.
Index	Automatically creates a unique index.	Index is not automatically created (optional).
Relation	Does not refer to any other table.	References a primary key in another table.
Example	student_id in Students table.	dept_id in Employees table referring to Departments table.

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Example to Understand

1. Primary Key Example

CREATE TABLE Students (
    student_id INT PRIMARY KEY,
    name VARCHAR(100),
    age INT
);

• Here, student_id is the Primary Key.
• It uniquely identifies each student.

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2. Foreign Key Example

CREATE TABLE Departments (
    dept_id INT PRIMARY KEY,
    dept_name VARCHAR(50)
);

CREATE TABLE Employees (
    emp_id INT PRIMARY KEY,
    emp_name VARCHAR(100),
    dept_id INT,
    FOREIGN KEY (dept_id) REFERENCES Departments(dept_id)
);

• In Employees table:
• emp_id is the Primary Key.
• dept_id is the Foreign Key that references the dept_id in the Departments table.
• It links each employee to a valid department.

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Key Points to Remember

• Primary Key ensures row uniqueness inside a table.
• Foreign Key ensures data consistency across two tables.
• Primary Key values must exist; Foreign Key values may or may not exist (if NULL is allowed).
• A Primary Key is mandatory for record identity; a Foreign Key is used when linking tables.

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Quick Real-Life Analogy

Real-World Example	Database Concept
Aadhaar Number (unique for every citizen)	Primary Key
Passport having Aadhaar number linked (referring Aadhaar database)	Foreign Key