有限公司相關MySQL中嵌套查詢

Question

架構：

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) 

問題：

P1。查找教職員的姓名，授課人數最多不超過 的部門。

P2。發現有招生比5

我嘗試更多的所有類的名字和他們的招生力量：

#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，這是給我的錯誤。請幫助我。

Answer 1

，Q＃1：只是做一個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 

使用相關查詢：

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

對於P2，您可以使用連接完成此操作，並且不需要嵌套查詢。在WHERE子句中，僅在HAVING子句中不允許對集合函數執行條件。您還必須先將班級名稱分組。

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; 

做一個聯合相關嵌套查詢，試試這個：

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