SELECT tag,SUM(a.if_retweet) as retweet_cut, ROUND(SUM(a.if_retweet)/count(start_time),3) as retweet_rate
from tb_user_video_log a JOIN tb_video_info b USING(video_id)
WHERE DATEDIFF((select MAX(start_time) from tb_user_video_log),start_time)<=29
GROUP BY tag
ORDER BY retweet_rate desc