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首页 > 解决方案 > 使用基于 Avg 的 Max 和 Min 来确定最低平均成本和最高平均成本时出错（MySQL）

mysql - 使用基于 Avg 的 Max 和 Min 来确定最低平均成本和最高平均成本时出错（MySQL）

问题描述

我根据以下标准确定了平均科目费用：

FROM BOOK
JOIN
(SELECT BOOK_SUBJECT,
Avg(BOOK_COST) AS AVGCOST
FROM BOOK
GROUP BY BOOK_SUBJECT) AS SUB USING (BOOK_SUBJECT)
ORDER BY BOOK_TITLE;

但是，我现在需要显示一个学科内书籍的最低平均成本和一个学科内书籍的最高平均成本。我知道我需要使用 MIN 和 MAX，但现在我必须基于 AVG 来确定语法。

这是我的尝试......

SELECT BOOK_SUBJECT
FROM BOOK
WHERE BOOK_COST =
(SELECT MIN(AVGCOST)
FROM BOOK)
ORDER BY BOOK_SUBJECT;

SELECT BOOK_SUBJECT
FROM BOOK
WHERE BOOK_COST =
(SELECT MAX(AVGCOST)
FROM BOOK)
ORDER BY BOOK_SUBJECT;

这会创建一个 SQL 错误：字段列表中的未知列“AVGCOST”。我该如何解决这个问题？我知道它正在寻找一个属性并且找不到它，那么需要在代码中添加什么以便 MySQL 计算平均值的最小值和最大值？非常感谢那些回复的人，您的反馈非常好。

标签： mysqlmaxaveragemin

解决方案

我认为你想要实现的是

SELECT *
FROM BOOK
JOIN
(
   SELECT BOOK_SUBJECT,
   AVG(BOOK_COST) AS AVGCOST,
   MIN(BOOK_COST) AS MINCOST,
   MAX(BOOK_COST) AS MAXCOST
   FROM BOOK
   GROUP BY BOOK_SUBJECT
) AS SUB USING (BOOK_SUBJECT)
ORDER BY BOOK_TITLE;

基于

BOOK_SUBJECT BOOK_TITLE BOOK_COST
------------ ---------- ---------------------
MATHS        Book 1     15.99
MATHS        Book 2     14.99
MATHS        Book 3     13.99
ENGLISH      Book A     10.99
ENGLISH      Book B     9.99

退货

BOOK_SUBJECT BOOK_TITLE BOOK_COST AVGCOST MINCOST MAXCOST
------------ ---------- --------- ------- ------- -------
MATHS        Book 1     15.99     14.99   13.99   15.99
MATHS        Book 2     14.99     14.99   13.99   15.99
MATHS        Book 3     13.99     14.99   13.99   15.99
ENGLISH      Book A     10.99     10.49   9.99    10.99
ENGLISH      Book B     9.99      10.49   9.99    10.99

仅提取 MIN/MAX 平均成本（四舍五入到小数点后 2 位）

SELECT 
  ROUND(MIN(MM.AVGCOST), 2) AS MINAVG, 
  ROUND(MAX(MM.AVGCOST), 2) AS MAXAVG
FROM
(
   SELECT BOOK_SUBJECT, 
   AVG(BOOK_COST) AS AVGCOST
   FROM BOOK
   GROUP BY BOOK_SUBJECT
) MM

将返回

MINAVG    MAXAVG
------    ------
10.49     14.99

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