hive 嵌套查询问题?

合并a.month,a.day,sum(a.pv)/count(a.)报错TOK_ALLCOLREF is not supported in current context：
Select a.month,a.day,sum(a.pv)/count(a.) from (Select month,day, remote_addr,count(1) as pv from ods_weblog_detail group by remote_addr,month,day) as a; ｛报错：TOK_ALLCOLREF is not supported in current context｝
单独分开a.month,a.day和sum(a.pv)/count(a.)都能正确执行：
Select sum(a.pv)/count(a.) from (Select month,day, remote_addr,count() as pv from ods_weblog_detail group by remote_addr,month,day) as a; 可以正确执行
Select a.month,a.day from (Select month,day, remote_addr,count() as pv from ods_weblog_detail group by remote_addr,month,day) as a; 也能正确执行
请问是什么原因呢？怎样解决啊？谢谢了！