text: DROP TABLE IF EXISTS `teachers`;
  CREATE TABLE `teachers`(
  	teacher_id INT PRIMARY KEY NOT NULL,
      name VARCHAR(255) NOT NULL,
      manager_id INT 
  );
  
  INSERT INTO `teachers` VALUES 
  	(101, 'John', NULL),
      (102, 'Maya', 106),
      (103, 'Silvia', 106),
      (104, 'Ted', 105),
      (105, 'Mark', 101),
      (106, 'Greta', 101);
      
  ALTER TABLE `teachers`
  ADD CONSTRAINT fk_manager FOREIGN KEY (manager_id)
  REFERENCES teachers(teacher_id);