机构，通常都是有层级的，当机构编码没有规则的时候，仅通过 id和pid，就需要
通过递归查询来求得给定机构的所有下属代码：
create table tb(id varchar(3) , pid varchar(3) , name varchar(100));
insert into tb values('002' , 0 , '浙江省');
insert into tb values('001' , 0 , '广东省');
insert into tb values('003' , '002' , '衢州市');
insert into tb values('004' , '002' , '杭州市') ;
insert into tb values('005' , '002' , '湖州市');
insert into tb values('006' , '002' , '嘉兴市') ;
insert into tb values('007' , '002' , '宁波市');
insert into tb values('008' , '002' , '绍兴市') ;
insert into tb values('009' , '002' , '台州市');
insert into tb values('010' , '002' , '温州市') ;
insert into tb values('011' , '002' , '丽水市');
insert into tb values('012' , '002' , '金华市') ;
insert into tb values('013' , '002' , '舟山市');
insert into tb values('014' , '004' , '上城区') ;
insert into tb values('015' , '004' , '下城区');
insert into tb values('016' , '004' , '拱墅区') ;
insert into tb values('017' , '004' , '余杭区') ;
insert into tb values('018' , '011' , '金东区') ;
insert into tb values('019' , '001' , '广州市') ;
insert into tb values('020' , '001' , '深圳市') ;
--这里就可以看到004所有的下级
WITH RECURSIVE cte(id,name,pid) as
(
select a.id,a.name,a.pid from tb a where id='004'
union all
select k.id,k.name,k.pid from tb k inner join cte c on c.id = k.pid
)
select id,name,pid from cte order by pid ;
--要保存递归查询的数据，只需再建一张表
create table vt(id varchar(3) , name varchar(100));
insert into vt
WITH RECURSIVE cte(id,name,pid) as
(
select a.id,a.name,a.pid from tb a where id='004'
union all
select k.id,k.name,k.pid from tb k inner join cte c on c.id = k.pid
)
select id,name,pid from cte order by pid ;