Primary Key vs Unique Key vs Foreign Key:

In Simple Words:

• Primary Key: Uniquely identifies each row; strictly unique and cannot be NULL (one per table).
• Unique Key: Enforces unique values; allows NULL values.
• Foreign Key: Links a column to the Primary Key of another table to maintain referential integrity.

CREATE TABLE orders (
  id SERIAL PRIMARY KEY,                        -- Primary Key (NOT NULL & UNIQUE)
  order_number VARCHAR(50) UNIQUE NOT NULL,      -- Unique Key
  user_id INT REFERENCES users(id) ON DELETE CASCADE -- Foreign Key
);