select 
tag,
sum(if_retweet) as retweet_cut,
round(sum(if_retweet)/count(tb1.video_id),3) as retweet_rate
from tb_user_video_log tb1 join tb_video_info tb2 on tb1.video_id = tb2.video_id

where datediff((select max(start_time) from tb_user_video_log),start_time)<30

group by tb2.tag
order by retweet_rate desc

难懂就难懂在“什么叫做[最近]”，题目虽然解释了范围但是没有解释怎么定义“现在”，后来看了大佬的解析才发现“现在”就是最大的start_time。写的稍微复杂了些，毕竟我水平有限。