What is Primary key, Foreign key & Unique key?

1. PRIMARY KEY: A primary key is a unique identifier that can’t have any duplicates and it can’t be null.

CREATE TABLE employees (
    employee_id INT PRIMARY KEY,
    name VARCHAR(50)
);

1. FOREIGN KEY: A foreign key column is a way to establish a relationship between two tables.

-- Create the parent table
CREATE TABLE departments (
    department_id INT PRIMARY KEY,       -- Primary Key
    department_name VARCHAR(50) NOT NULL
);

-- Create the child table with a foreign key
CREATE TABLE employees (
    employee_id INT PRIMARY KEY,          -- Primary Key
    employee_name VARCHAR(50) NOT NULL,
    department_id INT,                    -- Foreign Key
    FOREIGN KEY (department_id) REFERENCES departments(department_id)
);

1. UNIQUE KEY: A unique key in SQL makes sure that all values in a column are different and no duplicates are allowed.

CREATE TABLE users (
    user_id INT UNIQUE,
    email VARCHAR(100) UNIQUE
);

Differences between Primary Key, Foreign Key, and Unique Key: