如果要使用一条sql语句获得结果，可以尝试以下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程序。

您可以尝试将平均值保存在变量中，然后使用以下选项进行选择：

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，但你可以想出一个办法。

中的第一个子查询 HAVING 子句不仅仅返回当前作者的计数，它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，应该与子查询联接。

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，标识符是用反引号引用的，而不是单引号。单引号用于创建文字字符串。