SELECT * FROM person WHERE department IN
(SELECT department_id FROM department WHERE department_id = 20006
UNION
(SELECT department_id FROM
  (SELECT  * FROM department ORDER BY parent_id,department_id) depart_sorted,
  (SELECT @pv := 20006) initialisation
WHERE find_in_set(parent_id,@pv)
AND length(@pv := concat(@pv,‘,‘,department_id))));

CREATE TABLE department
(
    id BIGINT(11) PRIMARY KEY NOT NULL AUTO_INCREMENT,
    department_id INT(11) NOT NULL,
    parent_id INT(11),
    department_level INT(2) NOT NULL,
    department_name VARCHAR(20),
    attend_group VARCHAR(20)
);