drop table if exists demo.tree_data;
create table demo.tree_data (
 id integer,
 code text,
 pid integer,
 sort integer
);

insert into demo.tree_data values(1, '中国', null, 1);
insert into demo.tree_data values(2, '四川', 1, 1);
insert into demo.tree_data values(3, '云南', 1, 2);
insert into demo.tree_data values(4, '成都', 2, 1);
insert into demo.tree_data values(5, '绵阳', 2, 2);	
insert into demo.tree_data values(6, '武侯区', 4, 1);
insert into demo.tree_data values(7, '昆明', 3, 1);	

-- API 如下
connectby(text relname, 			-- 表名称
  text keyid_fld, 			-- id字段
  text parent_keyid_fld		-- 父id字段	
  [, text orderby_fld ], 	-- 排序字段
  text start_with, 			-- 起始行的id值
  int max_depth				-- 树深度，0表示无限
  [, text branch_delim ])	-- 路径分隔符

-- 基本用法如下，必须通过AS子句定义返回的字段名称和类型
select * 
	from connectby('demo.tree_data', 'id', 'pid', 'sort', '1', 0, '~')
	as (id int, pid int, lvl int, branch text, sort int);
	
-- 查询结果
id | pid | lvl | branch | sort
----+-----+-----+---------+------
 1 | | 0 | 1 | 1
 2 | 1 | 1 | 1~2 | 2
 4 | 2 | 2 | 1~2~4 | 3
 6 | 4 | 3 | 1~2~4~6 | 4
 5 | 2 | 2 | 1~2~5 | 5
 3 | 1 | 1 | 1~3 | 6
 7 | 3 | 2 | 1~3~7 | 7
(7 rows)

-- 仅仅使用基本用法，只能查询出id的相关信息，如果要查询code等其他字段，就需要通过额外的join操作来实现。
select 
	t.id, n.code, t.pid, p.code as pcode, lvl, branch
from (
	select * from connectby('demo.tree_data', 'id', 'pid', 'sort', '1', 0, '~')
		as (id int, pid int, lvl int, branch text, sort int)
) as t
	left join demo.tree_data as n on (t.id = n.id)
	left join demo.tree_data as p on (t.pid = p.id)
order by t.sort ;	

 id | code | pid | pcode | lvl | branch
----+--------+-----+-------+-----+---------
 1 | 中国 | | | 0 | 1
 2 | 四川 | 1 | 中国 | 1 | 1~2
 4 | 成都 | 2 | 四川 | 2 | 1~2~4
 6 | 武侯区 | 4 | 成都 | 3 | 1~2~4~6
 5 | 绵阳 | 2 | 四川 | 2 | 1~2~5
 3 | 云南 | 1 | 中国 | 1 | 1~3
 7 | 昆明 | 3 | 云南 | 2 | 1~3~7
(7 rows)

-- 
with recursive cte as
(
 -- 先查询root节点 
 select
 id, code, pid, '' as pcode,
 code as branch
 from demo.tree_data where id = 1
 union all
 -- 通过cte递归查询root节点的直接子节点 
 select
 origin.id, origin.code, cte.id as pid, cte.code as pcode,
 cte.branch || '~' || origin.code
 from cte
 join demo.tree_data as origin on origin.pid = cte.id
)
select
 id,code, pid, pcode, branch, 
 -- 通过计算分隔符的个数，模拟计算出树形的深度
 (length(branch)-length(replace(branch, '~', ''))) as lvl
from cte;

-- 
 id | code | pid | pcode | branch  | lvl
----+--------+-----+-------+-----------------------+-----
 1 | 中国 | | | 中国   | 0
 2 | 四川 | 1 | 中国 | 中国~四川  | 1
 3 | 云南 | 1 | 中国 | 中国~云南  | 1
 4 | 成都 | 2 | 四川 | 中国~四川~成都 | 2
 5 | 绵阳 | 2 | 四川 | 中国~四川~绵阳 | 2
 7 | 昆明 | 3 | 云南 | 中国~云南~昆明 | 2
 6 | 武侯区 | 4 | 成都 | 中国~四川~成都~武侯区 | 3
(7 rows)

-- step 1 执行
 select
 id, code, pid, '' as pcode,
 code as branch
 from demo.tree_data where id = 1
 
-- 结果集和working table为
 id | code | pid | pcode | branch
----+------+-----+-------+--------
 1 | 中国 | | | 中国

-- step 2 执行递归，此时自引用cte中的数据是step 1的结果
 select
 origin.id, origin.code, cte.id as pid, cte.code as pcode,
 cte.branch || '~' || origin.code
 from cte
 join demo.tree_data as origin on origin.pid = cte.id
 
 -- 结果集和working table为
 id | code | pid | pcode | branch 
----+--------+-----+-------+---------------------
 2 | 四川 | 1 | 中国 | 中国~四川  
 3 | 云南 | 1 | 中国 | 中国~云南