Sql 选择满足所有连接图元条件的图元

我发现很难给这个问题加上一个合适的标题，但也许那是因为我不知道如何实现我的目标

我有以下数据库布局：

table:   folders
columns: folderId

table: files
columns: fileId, authorId

table: file_relation
columns: folderId, fileId

table: authors
columns: authorId, surname, firstName

我试图实现的是列出所有只包含作者XY（姓氏=X，名字=Y）文件的文件夹。我的方法是这样的：

SELECT f.folderId FROM `folders` f 
     INNER JOIN `file_relation` rel ON rel.folderId = f.folderId 
     INNER JOIN `files` fls ON fls.fileId = rel.fileId 
     INNER JOIN `authors` a ON a.authorId = fls.authorId 
WHERE a.surname = X and a.firstName = Y;

SELECT f.folderId,
    COUNT(fls.fileId) as filesCount,
    SUM(a.surname = 'X' and a.firstName = 'Y') as authorFilesCount
FROM `folders` f 
     INNER JOIN `file_relation` rel ON rel.folderId = f.folderId 
     INNER JOIN `files` fls ON fls.fileId = rel.fileId 
     INNER JOIN `authors` a ON a.authorId = fls.authorId 
GROUP BY f.folderId
HAVING filesCount = authorFilesCount;

---

现在，我列出了所有的文件夹，其中有来自上述作者的文件。但是，我现在如何限制它们仅在没有其他文件的情况下列出

最好使用聚合函数而不是子查询。查询将如下所示：

SELECT f.folderId FROM `folders` f 
     INNER JOIN `file_relation` rel ON rel.folderId = f.folderId 
     INNER JOIN `files` fls ON fls.fileId = rel.fileId 
     INNER JOIN `authors` a ON a.authorId = fls.authorId 
WHERE a.surname = X and a.firstName = Y;

SELECT f.folderId,
    COUNT(fls.fileId) as filesCount,
    SUM(a.surname = 'X' and a.firstName = 'Y') as authorFilesCount
FROM `folders` f 
     INNER JOIN `file_relation` rel ON rel.folderId = f.folderId 
     INNER JOIN `files` fls ON fls.fileId = rel.fileId 
     INNER JOIN `authors` a ON a.authorId = fls.authorId 
GROUP BY f.folderId
HAVING filesCount = authorFilesCount;

---

你可以使用where not exists你说的“没有其他文件”是什么意思？您想要的结果是作者有一个文件但不超过一个文件？谢谢，where not EXISTS工作了@亚历克斯·约翰逊：我要声明的是，在这个文件夹中没有其他文件不是由作者X Y创建的。