select a.classId
from (select classId,count(*) as cnt from 表结构 where sex='男' group by classId) a
,  (select classId,count(*) as cnt from 表结构 where sex='女' group by classId)  b
where a.classId=b.classId and a.cnt=b.cnt

------解决方案--------------------




SELECT classId  FROM TTH
GROUP BY classId HAVING SUM(IF(sex='男',1,0))=SUM(IF(sex='女',1,0))