MySQL + SQL · Lesson 17

Primary Key and Foreign Key

Primary Key

A primary key uniquely identifies each row in a table. It cannot be NULL and cannot repeat.
CREATE TABLE students (
  roll_no INT PRIMARY KEY,
  name VARCHAR(50)
);

Foreign Key

A foreign key is a column that refers to the primary key of another table, creating a link between them.
CREATE TABLE fees (
  fee_id INT PRIMARY KEY,
  roll_no INT,
  amount INT,
  FOREIGN KEY (roll_no) REFERENCES students(roll_no)
);
fees.roll_no must match an existing students.roll_no.

Quick Difference

Primary KeyForeign Key
Identifies a row uniquelyLinks to another table
Cannot be NULLCan be NULL
One per tableMany allowed

Summary

  • Primary key = unique, not null, one per table.
  • Foreign key = links tables, enforces referential integrity.
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