如果值大于平均值，MySQL 显示数据 [英] MySQL show data if value greater than average

问题描述

我有这个查询，我想选择或显示书籍数量高于平均水平的作者.

I have this query where I want to select or show authors whose number of books is above the average.

到目前为止，我得到的平均图书数量的查询如下:

So far, the query I got to get the average number of books is the following:

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

然后我怎样才能只选择(显示)那些书数高于平均水平的作者?

How can I then select (show) only those authors whose number of books is above the average?

此查询列出了作者及其书籍数量:

This query lists the authors with their number of books:

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

我试过了:

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

推荐答案

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.但是一个 SELECT 用作值只能返回一个值.

Your first subquery in the HAVING clause isn't returning just the count of the current author, it's returning the counts of all others. But a SELECT used as a value can only return one value.

相反，您应该加入子查询.

Instead, you should join with the subquery.

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，您使用反引号引用标识符，而不是单引号.单引号用于创建文字字符串.

BTW, in MySQL you quote identifiers using backticks, not single quotes. Single quotes are for creating literal strings.

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