text: DROP DATABASE IF EXISTS employee_db;
  CREATE DATABASE employee_db;
  
  USE employee_db;
  
  -- create department
  CREATE TABLE department (
    id INT NOT NULL AUTO_INCREMENT PRIMARY KEY,
    dept_name VARCHAR(30) NOT NULL
  );
  
  -- create role
  CREATE TABLE role (
      id INT NOT NULL AUTO_INCREMENT PRIMARY KEY,
      title VARCHAR(30) NOT NULL,
      salary VARCHAR(30) NOT NULL,
      department_id INT,
      FOREIGN KEY (department_id)
      REFERENCES department(id)
      ON DELETE SET NULL
  );
  
  -- create employee
  CREATE TABLE employee (
      id INT NOT NULL AUTO_INCREMENT PRIMARY KEY,
      first_name VARCHAR(30) NOT NULL,
      last_name VARCHAR(30) NOT NULL,
      role_id INT,
      manager_id INT,
      FOREIGN KEY (role_id) REFERENCES role(id) ON DELETE SET NULL 
  );