Lesson 6 / 28

Primary & Foreign Keys

How tables link to each other and stay consistent.

Primary key

A primary key uniquely identifies each row in a table — no duplicates, no NULLs.

Foreign key

A foreign key in one table points to a primary key in another, enforcing the relationship.

CREATE TABLE orders (
  id INT AUTO_INCREMENT PRIMARY KEY,
  customer_id INT NOT NULL,
  order_date DATE,
  FOREIGN KEY (customer_id) REFERENCES customers(id)
);

Like a library ticket

A foreign key is like a library slip that references a book's catalog number instead of copying the whole book's details onto every slip.