select tag,sum(if_retweet) as retweet_cut,
round(sum(if_retweet)/count(*),3) as retweet_rate

from tb_user_video_log join tb_video_info using(video_id)    
    where datediff('2021-10-1',start_time)<30
    group by tag
    order by retweet_rate desc

重点在于理解题目中给到的实践范围。

datediff('2021-10-31','2021-10-2') =29

所以在从2021-10-1号往前推，时间间隔是小于30，不是<=30

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