Question

我有两张桌子stablestable和付款我想获取在特定班级中按特定学期付款的学生人数这是我的代码

SELECT classform, student_id, COUNT(student_id) as countreport_class
FROM studentstable s JOIN
     (SELECT studentid
      FROM payment
      WHERE term= ? AND acayear=?
     ) AS pa
     ON pa.studentid = s.student_id AND s.classform=? 
GROUP BY s.student_id

但是结果是错误的。而是显示学生付款的次数。我需要帮助。预先感谢。

Answer 1

我期望这样的事情：

SELECT COUNT(*) as countreport_class
FROM studentstable s JOIN
     payment p
     ON p.studentid = s.student_id AND
        p.term = ? AND p.acayear = ?
WHERE s.classform = ? ;

如果您希望查询返回单个数字，为什么要在select中返回那么多列？

以上计算了付款次数。如果您希望获得付款的学生人数，请使用exists：

SELECT COUNT(*) as countreport_class
FROM studentstable s 
WHERE EXISTS (SELECT 1
              FROM payment p
              WHERE p.studentid = s.student_id AND
                    p.term = ? AND p.acayear = ?
             ) AND
      s.classform = ? ;

Answer 2

此查询

SELECT distinct studentid
FROM payment
WHERE term = ? AND acayear = ?

返回此学期至少付款1次的所有学生。
因此，如果将students表加入该查询并应用特定班级的条件，您将具有所需的条件：

SELECT COUNT(*) as countreport_class
FROM studentstable s INNER JOIN (
  SELECT distinct studentid
  FROM payment
  WHERE term = ? AND acayear = ?
) AS pa ON pa.studentid = s.studentid 
WHERE s.classform = ?

请参见demo

如何获取在特定时期内付款的客户数量

2 个答案: