使用基于平均的最大和最小值来确定最低平均成本和最高平均成本时出错（MySQL）

Question

我已根据以下各项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主题中最低的书籍平均成本，以及一个主题中最高的平均书籍成本。我知道我需要使用MIN和MAX，但现在不确定语法，因为我必须基于AVG。

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;

这将产生一个SQL错误：字段列表中的未知列“ AVGCOST”。我该如何解决？我了解它正在寻找属性，但找不到它，因此需要在代码中添加什么内容，以便MySQL计算平均值和最小值？非常感谢那些回答，您的反馈非常棒。

Answer 1

我认为您想要实现的目标是

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

返回

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平均费用（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

会回来

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