仅显示等于临时表中最大值的实例

Question

我试图创建一个新表，其中只有sname的最大值等于num_courses这是临时表n中的一列。

SELECT s.sname, n.num_courses 
FROM (SELECT e.sid, COUNT(distinct e.cno) as num_courses 
     FROM enroll e 
     GROUP BY e.sid) n, student s 
WHERE s.sid = n.sid AND n.num_courses = (SELECT MAX(n.num_course) from n) x; 

是否有可能只显示等于在临时表中找到的最大值的实例？ （参见第二WHERE条款的最后一行）

这是错误：

ERROR 1064 (42000) at line 1 in file: 'q7.sql': You have an error in your 
SQL syntax; check the manual that corresponds to your MySQL server version 
for the right syntax to use near 'x' at line 5 

它说的错误是在第1行，但是当我删除最后一行的最后一句，有没有错误。

Answer 1

你不能像这样重用子查询。您需要在where子句中再次写入查询。

我想你打算这样做：

select s.sname, 
    n.num_courses 
from (
    select e.sid, 
     COUNT(distinct e.cno) as num_courses 
    from enroll e 
    group by e.sid 
    ) n, 
    student s 
where s.sid = n.sid 
    and n.num_courses = (
     select MAX(n.num_course) 
     from (
      select COUNT(distinct e.cno) as num_courses 
      from enroll e 
      group by e.sid 
      ) t 
     ); 

Answer 2

你并不需要一个别名WHERE条件，并尝试使用IN因为代替MAX()可能返回多个结果像

n.num_courses IN (SELECT MAX(n.num_course) from n); 

Answer 3

你不需要x表名，你不能使用n表别名，但你需要所有的代码

SELECT s.sname, n.num_courses 
    FROM (SELECT e.sid, COUNT(distinct e.cno) as num_courses 
     FROM enroll e 
     GROUP BY e.sid) n 
    INNER JOIN student s ON s.sid = n.sid AND (SELECT MAX(n.num_course) from 
    (SELECT e.sid, COUNT(distinct e.cno) as num_courses 
     FROM enroll e 
     GROUP BY e.sid) n)