代码之家 › 专栏 › 技术社区 › Sergey

sql HAVING max(count())返回零行

having max count sql mysql

Sergey · 技术社区 · 11 年前

我正在尝试获得课程表重叠的教室,我的桌子: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

类_房间:

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(目前为1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;

但返回的是空表。怎么了?或者,也许是另一个解决方案的建议,以获得我需要的东西?

2 回复 | 直到 11 年前

Lieven Keersmaekers 11 年前

以下内容将返回匹配最大计数的所有组

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
                    )

dnoeth 11 年前

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
     )

现在加入到 class_room 获得 location