找出班级里面男同学和女同学数量相同的班级

select male.cid, female.cid  from (select t1.class_id cid, count(1) total          from t_class t1         where t1.sex = 'male'         group by t1.class_id) male,       (select t2.class_id cid, count(1) total          from t_class t2         where t2.sex = 'female'         group by t2.class_id) female where male.total = female.total   and male.cid = female.cid;

两种写法 两种思路 但是第二种的写法会存在问题 考虑不全面 记下先