SELECT
    tag,
    SUM(if_retweet) AS retweet_cut,
    ROUND(SUM(if_retweet)/COUNT(video_id),3) AS retweet_rate
FROM tb_user_video_log log
JOIN tb_video_info info USING(video_id)
WHERE DATEDIFF((SELECT MAX(end_time) FROM tb_user_video_log),start_time)<=29
GROUP BY tag
ORDER BY retweet_rate DESC