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如下需求：
将左边父子结构转换成右边层级结构（转换到三个层级即可）。
难点解析：父子层级表难点主要在层级深度，父子结构解析需要自连接及id=parent_id逐步进行处理，第一次连接可以获取到两个层级，当第二次自连接即可获取到第三个层级
--1、当前为层级固定时 --2、层级不固定时 ...较为复杂 with area_info as (select 1 as id, '北京市' as
name, 0 as parent_id union all select 2 as id, '山东省' as name, 0 as parent_id
union all select 3 as id, '昌平区' as name, 1 as parent_id union all select 4 as
id, '海淀区' as name, 1 as parent_id union all select 5 as id, '沙河镇' as name, 3 as
parent_id union all select 6 as id, '马池口镇' as name, 3 as parent_id union all
select 7 as id, '中关村' as name, 4 as parent_id union all select 8 as id, '上地' as
name, 4 as parent_id union all select 9 as id, '烟台市' as name, 2 as parent_id
union all select 10 as id, '即墨区' as name, 9 as parent_id union all select 11 as
id, '牟平区' as name, 9 as parent_id ), P1 AS (SELECT p.ID LEVEL_1_ID, p.NAME
LEVEL_1_NAME, s.ID LEVEL_2_ID, s.NAME LEVEL_2_NAME, NULL LEVEL_3_ID, NULL
LEVEL_3_NAME, NULL LEVEL_4_ID, NULL LEVEL_4_NAME FROM area_info s --子 JOIN
area_info p --父 ON s.parent_id = p.id) , P2 AS ( SELECT p.LEVEL_1_ID,
p.LEVEL_1_NAME, p.LEVEL_2_ID, p.LEVEL_2_NAME, s.ID LEVEL_3_ID, s.NAME
LEVEL_3_NAME, NULL LEVEL_4_ID, NULL LEVEL_4_NAME FROM area_info s JOIN P1 p ON
(s.parent_id = p.LEVEL_2_ID) /* UNION ALL SELECT * FROM P1*/ ) SELECT
LEVEL_1_NAME,LEVEL_2_NAME,LEVEL_3_NAME FROM P2;
