题解 | #每类视频近一个月的转发量/率#
注意where中不能直接写聚合函数，而要使用子查询
日期是-29而不是-30
select b.tag, sum(a.if_retweet) as retweet_cut, round(sum(a.if_retweet)/count(*),3) as retweet_rate from tb_user_video_log as a left join tb_video_info as b on a.video_id=b.video_id where date(a.start_time)>=(select date_add(max(date(start_time)),interval -29 day) from tb_user_video_log) group by b.tag order by retweet_rate DESC
