MySql通过父id递归向下查询子节点

SELECT
  ID.LEVEL,
  DATA.* 
FROM
  (
  SELECT
    @ids AS _ids,
    ( SELECT @ids := GROUP_CONCAT( region_id ) FROM region WHERE FIND_IN_SET(parent_id, @ids ) ) AS cids,
    @l := @l + 1 AS LEVEL 
  FROM
    region,
    ( SELECT @ids := 3, @l := 0 ) b 
  WHERE
    @ids IS NOT NULL 
  ) ID,
  region DATA 
WHERE
  FIND_IN_SET( DATA.region_id, ID._ids ) 
ORDER BY
  LEVEL

--创建测试环境
create table t_test(
  id int PRIMARY key,
  parent_id int,
  name varchar(200)
)
insert t_test VALUES(1,null,"中国");
insert t_test VALUES(2,1,"华北");
insert t_test VALUES(3,2,"山西省");
insert t_test VALUES(4,2,"北京");
insert t_test VALUES(5,3,"临汾市");
insert t_test VALUES(6,4,"北京市");
insert t_test VALUES(7,5,"尧都区");
insert t_test VALUES(8,6,"朝阳区");
insert t_test VALUES(9,7,"解放西路");
insert t_test VALUES(10,8,"朝阳北路");
SELECT * FROM t_test;

SELECT
  ID.LEVEL,
  DATA.* 
FROM
  (
  SELECT
  @ids AS _ids,
  ( SELECT @ids := GROUP_CONCAT( id ) FROM t_test WHERE FIND_IN_SET(parent_id, @ids ) ) AS cids,
  @l := @l + 1 AS LEVEL 
  FROM
  t_test,
  ( SELECT @ids := 1, @l := 0 ) b 
  WHERE
  @ids IS NOT NULL 
  ) ID,
  t_test DATA 
WHERE
  FIND_IN_SET( DATA.id, ID._ids ) 
ORDER BY
  LEVEL

SELECT
  ID.LEVEL,
  DATA.* 
FROM
  (
  SELECT
  @ids AS _ids,
  ( SELECT @ids := GROUP_CONCAT( id ) FROM t_test WHERE FIND_IN_SET(parent_id, @ids ) ) AS cids,
  @l := @l + 1 AS LEVEL 
  FROM
  t_test,
  ( SELECT @ids := 3, @l := 0 ) b 
  WHERE
  @ids IS NOT NULL 
  ) ID,
  t_test DATA 
WHERE
  FIND_IN_SET( DATA.id, ID._ids ) 
ORDER BY
  LEVEL