**自关联，表：省份、城市、县级区都有id、name共同属性，可以建立一张卡， id、name、pid ** 查询山东省有哪些城市
select province.name,city.name from areas as province inner join areas as city on city.pid=provice.id having provice.name='山东省';
子查询（速度慢点）
查询最高男生信息 select * from student where height=(select max(height) from student );
