Question

我发现很难用合适的标题来标记这个问题，但也许这就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

我尝试实现的是列出仅包含作者X Y（姓氏= X，firstName = Y）的文件的所有文件夹。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;

现在列出了所有来自所述作者的文件的文件夹。但是，如果没有其他文件，我现在如何限制它们才被列出？

Answer 1

最好使用聚合函数而不是子查询。查询将如下所示：

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;

选择所有连接元素满足条件的元素

1 个答案: