1. 计算一个表中某个字段值在表中的占比
场景：
table: frequency
field:adv_id, user_id，dis_imp_pv, imp_pv,freq, uv
计算客户id为25的访客中竞价id去重浏览次数在10次以上的用户数占比
select a.adv_id,sum(a.pv) asTOTAL,avg(b.total_pv),
sum(a.pv)/avg(b.total_pv) as freq
from frequency a,
(select adv_id,sum(pv) as total_pv
from frequency b where adv_id=25
group by adv_id) b
where a.adv_id=b.adv_id
and a.advr_id=25 and cast(a.imp_pv as int)>= 11
group by a.adv__id;
相当于在原表中增加了一列新的总数列
可以优化，使用case when 语句
SELECT
sum(case when imp_pv > 3 then pv else 0end)/sum(pv)
from frequency WHERE advr_id=25;
简单整洁并且高效
