text: DROP DATABASE IF EXISTS employee_db;
  CREATE DATABASE employee_db;
  
  USE employee_db;
  
  -- create tables --
  
  -- department --
  DROP TABLE IF EXISTS department;
  CREATE TABLE department(
    id INT NOT NULL AUTO_INCREMENT PRIMARY KEY,
    name VARCHAR(30) NOT NULL
  );
  
  -- role
  DROP TABLE IF EXISTS role;
  CREATE TABLE role (
    id INT NOT NULL AUTO_INCREMENT PRIMARY KEY,
    title VARCHAR(30) NOT NULL,
    salary DECIMAL,
    department_id INT,
  
    CONSTRAINT fk_role_department
      FOREIGN KEY (department_id) 
      REFERENCES department(id)
      ON DELETE SET NULL
  );
  
  -- employee
  DROP TABLE IF EXISTS 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,
  
    CONSTRAINT fk_employee_manager
      FOREIGN KEY (manager_id) 
      REFERENCES employee(id)
      ON DELETE SET NULL,
    CONSTRAINT fk_employee_role
      FOREIGN KEY (role_id) 
      REFERENCES role(id)
      ON DELETE SET NULL
  );