9-自关联

区域表：

01.png

1.查询河北省下面所有的市。

select * from areas as shi inner join areas as sheng on shi.pid=sheng.id where sheng.name="河北省";

02.png

2.查询张家口市下面所有的县。

select * from areas as shi inner join areas as xian on shi.id=xian.pid where shi.name="张家口市";

03.png