mysql CONCAT 在 WHERE 之后？答案

【问题标题】：mysql CONCAT after WHERE?mysql CONCAT 在 WHERE 之后？
【发布时间】：2025-12-05 19:40:02
【问题描述】：

我有一个管理考勤的查询。我想添加一个功能，显示谁支付了当月的费用，谁没有支付。它几乎可以工作。

SELECT
    students.sid,
    students.name,
    students.day,
    students.times,
    students.days,
    CONCAT(payments.year,'-', payments.forMonth) AS pdate
FROM students
LEFT JOIN payments
    ON students.sid = payments.sid
WHERE
    students.Active = 'Yes' AND
    students.day LIKE 'Tue%' AND
    payments.date = (SELECT date FROM payments WHERE students.sid = payments.sid AND
                     payments.payfor = 'tuition' ORDER BY payments.pid DESC LIMIT 1 )

主要的麻烦部分是 (SELECT date FROM) 子查询。 CONCAT 在 payments 子查询 集合中的 payments.payfor 限制器之前从 payments 表中获取第一个值。因此，在下面的表 sn-p 中，我得到了 sk1 条目，而不是 Dec

的正确 tuition

pid     sid  amt    payfor  forMonth    year    date
1076    69  7000    tuition Dec         2017    2017-12-17
1074    69  4000     sk1    Sep         2017    2017-12-17
1046    69  7000    tuition Nov         2017    2017-11-23

有没有办法在 CONCAT(payments.year,'-', payment.forMonth) 之前或同时获取 payments.date 子查询？

【问题讨论】：

标签： mysql subquery concat

【解决方案1】：

好的，我明白了。我加了

AND payments.payfor = 'tuition'

之后

students.Active = 'Yes' AND
students.day LIKE 'Tue%'

在我原来的查询中。然后，一切都按需要进行。

【讨论】：

【解决方案2】：

您应该在相关的 on 子句中移动与左连接表相关的条件，否则如果在 where 条件中作为内连接工作并且不返回相关行

  SELECT students.sid, students.name, students.day, students.times, students.days, 
  CONCAT(payments.year,'-', payments.forMonth) AS pdate 
  FROM students 
  LEFT JOIN payments ON students.sid = payments.sid  
        AND payments.date = (SELECT date 
          FROM payments 
          WHERE students.sid = payments.sid 
          AND payments.payfor = 'tuition' 
          ORDER BY payments.pid DESC LIMIT 1 
        )
  WHERE students.Active = 'Yes' 
  AND students.day LIKE 'Tue%'

如果你只想要学费，你也应该过滤这个值

  SELECT students.sid, students.name, students.day, students.times, students.days, 
  CONCAT(payments.year,'-', payments.forMonth) AS pdate 
  FROM students 
  LEFT JOIN payments ON students.sid = payments.sid  
        AND payments.date = (SELECT date 
          FROM payments 
          WHERE students.sid = payments.sid 
          AND payments.payfor = 'tuition' 
          ORDER BY payments.pid DESC LIMIT 1 
        ) 
        AND payments.payfor='tuition'

  WHERE students.Active = 'Yes' 
  AND students.day LIKE 'Tue%'

【讨论】：

谢谢，但它仍在获取所有的“payments.payfor”，而不仅仅是“学费”
我猜我们是同时找到的。