sql HAVING max(count()) 返回零行

2021-12-30 00:00:00 max sql count mysql having

我正在尝试使用重叠课程安排的教室，我的表格:课程:

I'm trying to get class rooms with overlap course schedule, my tables: courses:

COURSE_ID    NAME
11           matematika
22           logika
33           himiya
44           sport
55           algoritmika
66           hedva
77           algebra linearit

日程:

ID  COURSE_ID  ID_ROOM  DAY  HOUR
1   11         105      Mon  10am
2   11         105      Wen  10am
3   11         105      Thu  10am
4   22         105      Mon  10am
5   22         205      Wen  10am
6   22         105      Thu  10am
7   33         305      Mon  11am
8   33         105      Mon  10am

教室:

ID_ROOM  LOCATION  CAPACITY
105      A         20
205      B         10
305      C         30

我的sql是:

select class_room.ID_ROOM as crid, class_room.LOCATION, schedule.DAY as d, schedule.HOUR as h,  count(courses.COURSE_ID) as count 
  from schedule
  natural join class_room
  natural join courses
  group by crid, d, h
  order by count desc;

我得到:

crid  LOCATION  d   h       count
105   A         Mon 10am    3
105   A         Thu 10am    2
305   C         Mon 11am    1
105   A         Wen 10am    1
205   B         Wen 10am    1

但我只需要显示计数的所有最大值(现在有 1 个这样的行).我试过了

But I need to show all maximal values of count only (1 such row for now). I tryed 

select class_room.ID_ROOM as crid, class_room.LOCATION, schedule.DAY as d, schedule.HOUR as h,  count(courses.COURSE_ID) as count 
  from schedule
  natural join class_room
  natural join courses
  group by crid, d, h
  having max(count)
  order by count desc;

但是是返回空表.怎么了?或者，也许是另一种解决方案的建议，以获得我需要的东西?

But is return empty table. What is wrong? Or, maybe suggestion of another solution, to get what I need?

推荐答案

以下将返回匹配最大数量的所有组

Following would return all groups matching the max count

SQL 小提琴

select  class_room.ID_ROOM as crid
        , class_room.LOCATION
        , schedule.DAY as d
        , schedule.HOUR as h
        ,  count(courses.COURSE_ID) as count 
from    schedule
        natural join class_room
        natural join courses
group by 
        crid, d, h
having count(*) = (
                    select  max(count)
                    from    (            
                              select  count(courses.COURSE_ID) as count
                              from    schedule
                                      natural join class_room
                                      natural join courses
                              group by 
                                      id_room, day, hour
                            ) maxcount
                    )

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