INSERT INTO mnu_role_task (role_id, task_id) 
     SELECT mnu_role.role_id, 'mnu_motd(show)'     
       FROM mnu_role 
      WHERE NOT EXISTS (SELECT 1 FROM mnu_role_task AS mrt 
                         WHERE mrt.role_id=mnu_role.role_id AND mrt.task_id='mnu_motd(show)');