mysql - #1111 SQL 中 Group 函数的使用无效 : Concept?

Question

我正在尝试运行以下 SQL 代码，该代码会生成过去 3 天的平均问题。但是，我收到 #1111 invalid use of group function 错误。不知道我哪里出错了。即使我删除了该组，这仍然存在。请帮忙。

select BookId, ChapterName, AuthorCountry,
       avg(sum(No_of_issues)) as last3dayavgissues
from library_data
where Date between (select max(Date) from library_data) and
                   (select (max(Date)-3) from library_data) 
group by BookId, ChapterName 
having (AuthorCountry='US' OR AuthorCountry='DE')) 
group by BookId, ChapterName 
order by AuthorName
into outfile 'C:/output files/abc.csv'
        fields terminated by '\t'
        lines terminated by '\n';

谢谢 视频

Answer 1

在 avg 函数中使用 sum 函数会导致组函数无效使用错误。sum 函数仅返回一个值，单个值的平均值没有意义。