Question

我在此查询中要选择或显示图书数量高于平均水平的作者。

到目前为止，我要获取平均图书数量的查询如下：

SELECT AVG(c.quantity) AS 'Average'
FROM(
SELECT al.autores_DNI AS 'Author',COUNT(*) AS 'quantity'
FROM autores_has_libros AS al
GROUP BY al.autores_DNI
) AS c

然后我该如何仅选择（显示）书籍数量高于平均水平的作者？

此查询列出了作者及其书数：

SELECT al.autores_DNI AS 'Author',COUNT(*) AS 'quantity'
    FROM autores_has_libros AS al
    GROUP BY al.autores_DNI

我尝试过：

SELECT a.first_name 
FROM autores AS a
HAVING (
SELECT al.autores_DNI AS 'Author',COUNT(*) AS 'quantity'
    FROM autores_has_libros AS al
    GROUP BY al.autores_DNI
)
> (
SELECT AVG(c.quantity) AS 'Average'
    FROM(
    SELECT al.autores_DNI AS 'Author',COUNT(*) AS 'quantity'
    FROM autores_has_libros AS al
    GROUP BY al.autores_DNI
    ) AS c
)

但这不起作用

Answer 1

您在HAVING子句中的第一个子查询不仅返回当前作者的计数，还返回所有其他作者的计数。但是SELECT用作值只能返回一个值。

相反，您应该加入子查询。

SELECT a.first_name
FROM aotores AS a
JOIN (
    SELECT al.autores_DNI AS Author,COUNT(*) AS quantity
    FROM autores_has_libros AS al
    GROUP BY Author) AS al ON al.Author = a.DNI
JOIN (
    SELECT AVG(c.quantity) AS Average
    FROM (
        SELECT COUNT(*) AS quantity
        FROM autores_has_libros AS al
        GROUP BY al.autores_DNI) AS c
) AS av ON al.quantity > av.Average

顺便说一句，在MySQL中，您使用反引号而不是单引号来引用标识符。单引号用于创建文字字符串。

Answer 2

您可以尝试将平均值保存在变量中，然后执行选择：

Set @avg = SELECT AVG(c.quantity) AS 'Average'
FROM(
SELECT al.autores_DNI AS 'Author',COUNT(*) AS 'quantity'
FROM autores_has_libros AS al
GROUP BY al.autores_DNI
) AS c

SELECT al.autores_DNI AS 'Author',COUNT(*) AS 'quantity'
FROM autores_has_libros AS al where COUNT(*) >= @avg

我不确定这是否行得通，但是您可以对此有所了解。

Answer 3

如果要使用一个sql语句获取结果，可以尝试使用以下sql：

SELECT a.first_name FROM autores AS a 
RIGHT JOIN 
(
    SELECT autores_DNI AS Author, COUNT(*) AS quantity
    FROM autores_has_libros WHERE quantity > (
        SELECT AVG(quantity) AS Average FROM
        (
            SELECT autores_DNI AS Author, COUNT(*) AS quantity
            FROM autores_has_libros
            GROUP BY autores_DNI
        )
    )
    GROUP BY autores_DNI
) b ON a.author = b.Author

但是，我建议您可以使用语言代码在比较之前获得平均值，或者使用mysql过程。

如果值大于平均值，则MySQL显示数据

3 个答案: