题解 | #每篇文章同一时刻最大在看人数#

这一题是经典的时间戳内最大的在线人数类问题，但注意最大的坑点在于“同一个时间点先统计进再统计出”这个逻辑，比如在t时刻有3条行为，进、进、出；如果按照先进后出，1、1、-1 对应的sum() over()的当前累积和结果是1,2,1；如果是-1、1、1，则是

-1、0、1；显然当前累积和会受到用户进出行为状态以及排序的影响

select artical_id,
            max(cnt) max_uv
    from
    (select 
        artical_id,
        time,
        sum(log) over(partition by artical_id order by time asc,log desc) cnt
from
    (select 
        artical_id,
        uid,
        in_time time,
        1 log
    from tb_user_log
    where artical_id !=0
    group by 
        artical_id,
        uid,
       in_time
    
   union all
    select 
        artical_id,
        uid,
        out_time time,
        -1 log
    from tb_user_log
     where artical_id !=0
    group by 
        artical_id,
        uid,
        out_time) a 
    ) t1
        group by artical_id
        order by max(cnt) desc