Mysql选择4个表3个计数

Question

我有四张桌子

users
ID  display_name
1   Name1
2   Name2
3   Name3

用户可以将书籍添加到表格书

books
book_id AddedByuserID
1       1
2       1
3       2
4       3

用户也可以将电子书添加到表格电子书

ebooks
ebook_id AddedByuserID
1       1
2       2
3       2
4       3

现在，用户只能将书籍添加到他的馆藏（而不是电子书）

collection
userID  book_id
1       1
1       2
1       3

我需要这样的输出：

display_name    books_added ebooks_added    books_in_collection
Name1           2           1               3
Name2           1           2               2
Name3           1           1               1

这是得到的：

SELECT users.*, COUNT(DISTINCT collection.book_id) AS books_in_collection, COUNT(DISTINCT books.AddedByuserID) AS books_added, COUNT(DISTINCT ebooks.AddedByuserID) AS ebooks_added
FROM users LEFT JOIN collection ON users.ID = collection.userID
LEFT JOIN books ON users.ID = books.AddedByuserID
LEFT JOIN ebooks ON users.ID = ebooks.AddedByuserID
GROUP BY users.ID
ORDER BY display_name ASC

用户display_name显示正确，也显示集合计数，但另外两个计数显示为1。 如果我删除DISTINCT，则收集计数为86，而其他两个计数则不显示。

我尝试了解LEFT加入并阅读教程，但我现在陷入困境。

Answer 1

我建议使用相关子查询：

select u.*,
       (select count(*) from collection c where u.id = c.userId) as books_in_collection,
       (select count(*) from books b where u.id = b.AddedByUserId) as books_added,
       (select count(*) from ebooks e where u.id = e.AddedByUserId) as ebooks_added
from users u;

我不知道你为什么要过滤掉第三个用户。