if函数搞定 
select b.tag,concat(round(sum(if(timestampdiff(second,start_time,end_time)>duration,duration,timestampdiff(second,start_time,end_time)))/sum(duration)*100,2),'%') as rate
from tb_user_video_log a
join tb_video_info b
on a.video_id=b.video_id
group by b.tag
having replace(rate,'%','')>60
order by rate desc;