SQL189大厂面试真题类似SQL49题
统计直播各科目同时在线人数
核心代码如下:
select course_id, times, sum(tag)over(partition by course_id order by times)num from
(
select course_id, in_datetime as times, 1 as tag from attend_tb union all
select course_id, out_datetime as times, -1 as tag from attend_tb
order by course_id, times
)a