Self-join คืออะไร?

employees table — manager_id references another employee's id (in the SAME table):
  id | name  | manager_id
  1  | Ann   | NULL         (Ann is the boss)
  2  | Bob   | 1            (Bob's manager is Ann)
  3  | Carol | 1            (Carol's manager is Ann)

 e.name  employee, m.name  manager
 employees e
  employees m  e.manager_id  m.id;