题解 | #每类视频近一个月的转发量/率#

-- 有用户互动的最近时间：max(start_time)
-- 最近一个月：当天-29天

SELECT tag, 
    SUM(if_retweet) AS retweet_cut,
    ROUND(SUM(if_retweet) / COUNT(start_time), 3) AS retweet_rate
FROM tb_user_video_log 
    JOIN tb_video_info
    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