题解 | #每篇文章同一时刻最大在看人数#

select
    artical_id,
    max(uv) as max_uv
from
    (
        select
            artical_id,
            sum(tag) over (
                partition by
                    artical_id
                order by
                    dt, tag desc
            ) as uv
        from
            (
                select
                    artical_id,
                    in_time as dt,
                    1 as tag
                from
                    tb_user_log
                where
                    artical_id <> 0
                union all
                select
                    artical_id,
                    out_time as dt,
                    -1 as tag
                from
                    tb_user_log
                where
                    artical_id <>0
            ) as t1
    ) as t2
group by
    artical_id
order by
    max_uv desc;

对于t2子查询那里我有点不理解为啥要对tag desc，有没有大佬指点一下