SQL查询最大在线人数（通用方法）

标记进入时间为1，退出时间为-1，通过union all将两个标记好的表格合并成一张表格，通过sum()over()窗口函数，可以求出在线人数。最后通过max聚合函数，求出最大的在线人数。具体题目跟代码实现如下：

  SELECT artical_id,MAX(sums) max_uv
   FROM 
  (SELECT *,
  SUM(temp1.uv)over(PARTITION BY artical_id ORDER BY TIME,uv desc)  sums
  FROM 
  (SELECT uid,artical_id,in_time TIME, 1 uv FROM tb_user_log WHERE artical_id!=0
  UNION ALL
  SELECT uid,artical_id,out_time TIME, -1 uv FROM tb_user_log WHERE artical_id!=0) temp1) temp2
  GROUP BY artical_id
  ORDER BY max_uv DESC