Question

架构：

Student (snum: integer, 
         sname: char(30), 
         major: char(25), 
         level: char(2), 
         age: integer)

Faculty (fid: integer, 
         fname: char(30), 
         deptid: integer)

Class (cname: char(40), 
       meets_at: char(20),
       room: char(10), 
       fid: integer | fid    REFS Faculty.fid)

Enrolled (snum: integer, 
          cname: char(40) | snum REFS student.snum, 
          cname REFS class.name)

问题：

P1。找教师，教授最大人数的部门的名称不同的类。

P2。查找所有课程的名称及其注册人数超过5的注册人数。

我的尝试：

#P1:
select distinct f.fname,max(distinct c.cname)
from faculty f,class c
where Exists (select c.fid,max(distinct c.cname) as myCount
from class c where
f.fid=c.fid);

#P2:
select distinct c.cname
from class c
where Exists (select c.cname
from enrolled e where
e.cname=c.cname and count(e.cname)>5);

但是这给了我错误。请帮帮我。

Answer 1

问题＃1 ：只需group by

select f.fname, COUNT(distinct c.cname)
from faculty f 
join class c
  on f.fid = c.fid
group by f.fname
order by COUNT(distinct c.cname) DESC
limit 1

使用相关查询：

Corraleted v1：

SELECT *
FROM faculty f
WHERE f.id =  (SELECT c.fid
               FROM class c 
               GROUP BY c.fid
               ORDER BY COUNT(distinct c.cname) DESC
               LIMIT 1)

Corraleted v2：

SELECT *
FROM (
      SELECT f.*, (SELECT COUNT(distinct c.cname)
                   FROM class c
                   WHERE c.fid = f.id) as total
      FROM faculty f
     ) T
ORDER By Total DESC
LIMIT 1

Answer 2

对于P2，您可以使用连接执行此操作，而不使用嵌套查询。仅在HAVING子句中，WHERE子句中不允许对聚合函数执行条件。您还必须先对类名进行分组。

var map = new ALKMaps.Map("map");

for (var c in map.controls) {
    if (map.controls[c].CLASS_NAME == "ALKMaps.Control.KeyboardDefaults")
        map.controls[c].deactivate();
}

执行共同相关的嵌套查询，请尝试以下方法：

SELECT C.name, count(*) as enrollment
FROM Class C INNER JOIN Enrolled E
    ON C.cname=E.cname
GROUP BY C.name
HAVING count(*) > 5;

MySQL中与Co相关的嵌套查询

2 个答案: