with 子查询及递归的使用

```sql
create table t_area(id int,pid int,area varchar);
insert into t_area values(1,null,'广东省');
insert into t_area values(2,1,'深圳市');
insert into t_area values(3,1,'东莞市');
insert into t_area values(4,2,'南山区');
insert into t_area values(5,2,'福田区');
insert into t_area values(6,2,'罗湖区');
insert into t_area values(7,3,'南城区');
insert into t_area values(8,3,'东城区');

with t_gd_city AS (
  select * from t_area where pid=1
)
select * FROM t_gd_city;