Question

我正在努力让课堂上有重叠的课程安排，我的课程：课程：

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

时间表：

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;

我得到了：

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

但是我需要显示所有最大值的计数（现在只有1个这样的行）。我试过

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;

但是返回空表。怎么了？或者，也许建议另一种解决方案，以获得我需要的东西？

Answer 1

以下将返回与最大计数匹配的所有组

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
                    )

Answer 2

这将返回计数最高的所有行：

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
     )

现在将其加入class_room以获取location

sql HAVING max（count（））返回零行

2 个答案: