题解 | 牛客直播各科目同时在线人数

with t as(
    select  in_datetime dt,course_id, 1 as type from attend_tb
    union all
    select  out_datetime dt,course_id, -1 as type from attend_tb
)

select 
course_id,course_name,
max(num) as max_num
from(
    select c.course_id,course_name,
sum(type) over (partition by c.course_id order by dt,type desc) as num
from course_tb c
join t
on c.course_id = t.course_id
)b
group by course_id,course_name
order by course_id

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