sql 有 max(count()) return 零行

Question

我正在尝试获得 class 课程安排重叠的房间，我的 tables: 课程：

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

日程安排：

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

class_room:

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（现在只有 1 个这样的行）。我试过了

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return是空的table。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？

Answer 1

这将 return 具有最高计数的所有行：

select ID_ROOM as crid, DAY as d, HOUR as h,
      count(*) as cnt
from schedule
group by crid, d, h
having
   count(*)
   = ( select count(*) as cnt
       from schedule
       group by ID_ROOM, DAY, HOUR
       order by cnt desc
       limit 1
     )

现在加入 class_room 以获得 location

Answer 2

以下将 return 所有组匹配最大计数

SQL Fiddle

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
                    )

sql 有 max(count()) return 零行

sql HAVING max(count()) return zero rows

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