我正在运行以下hql：

hive>select period,count(*) from
(select case when hour(a.first_time)>=21 and hour(a.first_time)<24 then'21-24'
             when hour(a.first_time)>=12 and hour(a.first_time)<21 then'12-21'
             else '0-12'
             end as period
             from longyuan_web.tv_log as a ) p1
group by period;

但是什么也没回来，有人能告诉我为什么吗？
我期望输出如下：

0-12    34000
12-21   12000
21-24   32000

非常感谢你！