Sql count和min

Question

我对如何执行以下sql命令有疑问：

报告具有最低注册人数的课程。您应该输出课程代码和注册数量。

有问题的表： - 与CourseCode，SectionNo，StudentID和

级

的表格报名，我不知道从哪里开始使用此所以任何帮助将非常感激。

我想这完全不工作的情况如下：

select p.CourseCode, count(p.StudentID) AS Num_Students 
from Enrollment p 
where count(p.StudentID) = (select min(select count(e1.StudentID)) from Enrollment e1 where e1.CourseCode = p.CourseCode) 
group by p.CourseCode; 

我想的结果是有CourseCode和学生只能与StudentIDS的分额的课程数目。我认为我的逻辑很好，但我不太了解sql语法以获得我想要的。

Answer 1

获取与studentid的最小非零计数过程中（最低数招生）：

SELECT COUNT(e.studentid) AS lowest_nonzero_count 
    FROM Enrollment e 
GROUP BY e.coursecode 
ORDER BY 1 
LIMIT 1 

但是，如果我们要考虑的是没有任何的入学课程（即，如果我们考虑到零是最低的数字入学的有效值）

SELECT COUNT(e.studentid) AS lowest_count 
    FROM course c 
    LEFT 
    JOIN enrollment e 
    ON e.coursecode = c.coursecode 
GROUP BY c.coursecode 
ORDER BY 1 
LIMIT 1 

哪些课程有一个最低（非零）studentid的计数？如果我们知道具体的值，我们可以做到这一点（代表有一个占位符:lowest_nonzero_count这个值：

SELECT d.coursecode 
    , COUNT(d.studentid) 
    FROM enrollment d 
GROUP BY d.coursecode 
HAVING COUNT(d.studentid) = :lowest_nonzero_count 

这是我们能够与返回值的子查询替换:lowest_nonzero_count占位符：

SELECT d.coursecode 
    , COUNT(d.studentid) 
    FROM enrollment d 
GROUP BY d.coursecode 
HAVING COUNT(d.studentid) = 
     (SELECT COUNT(e.studentid) 
      FROM Enrollment e 
      GROUP BY e.coursecode 
      ORDER BY 1 
      LIMIT 1 
     ) 
ORDER BY d.coursecode  

如果我们要考虑一个“零”值作为入学率的有效号码：

SELECT b.coursecode 
    , COUNT(d.studentid) 
    FROM course b 
    LEFT 
    JOIN enrollment d 
    ON d.coursecode = b.coursecode 
GROUP BY b.coursecode 
HAVING COUNT(d.studentid) = 
     (SELECT COUNT(e.studentid) 
      FROM course c 
      LEFT 
      JOIN enrollment e 
      ON e.coursecode = c.coursecode 
      GROUP BY c.coursecode 
      ORDER BY 1 
      LIMIT 1 
     ) 
ORDER BY b.coursecode