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
