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Question

我有这些表：

用户

id_user | name 
1 | First 
2 | Second 

书

id_book | id_user | pages 
1 | 1 | 10 
2 | 1 | 5 
3 | 1 | 30 
4 | 2 | 15 
5 | 2 | 20 

电子书

id_ebook | id_user | pages 
1 | 1 | 20 
2 | 1 | 20 
3 | 2 | 10 
4 | 2 | 20 
5 | 2 | 10 

我想创建一个单个表格告诉我每个用户有多少书h电子书的总数，页数，电子书总数以及页面总数。

报告

id_user | total_books | sum_pag_books | total_ebooks | sum_pag_ebooks 
1 | 3 | 45 | 2 | 40 
2 | 2 | 35 | 3 | 40 

我有这样的代码：

SELECT 
user.name, 
COUNT(book.id_book) AS 'total_books', 
SUM(book.pages) AS 'sum_pag_books', 
COUNT(ebook.id_ebook) AS 'total_ebooks', 
SUM(ebook.pages) AS 'sum_pag_ebooks' 
FROM user 
LEFT JOIN book ON book.id_user = user.id_user 
LEFT JOIN ebook ON ebook.id_user = user.id_user 
GROUP BY user.id_user 

但是，结果是这样的，是不正确的，我该如何解决这个查询？ 报告

id_user | total_books | sum_pag_books | total_ebooks | sum_pag_ebooks 
1 | 6 | 90 | 6 | 120 
2 | 6 | 105 | 6 | 80 

Answer 1

你可以尝试分别聚集两个书表，然后加入到：

SELECT 
    user.name, 
    COALESCE(book.total_books, 0)  AS total_books, 
    COALESCE(book.sum_pag_books, 0) AS sum_pag_books, 
    COALESCE(ebook.total_ebooks, 0) AS total_ebooks, 
    COALESCE(ebook.sum_pag_ebooks, 0) AS sum_pag_ebooks 
FROM user 
LEFT JOIN 
(
    SELECT id_user, COUNT(*) AS total_books, SUM(pages) AS sum_pag_books 
    FROM book 
    GROUP BY id_user 
) book 
    ON book.id_user = user.id_user 
LEFT JOIN 
(
    SELECT id_user, COUNT(*) AS total_ebooks, SUM(pages) AS sum_page_ebooks 
    FROM ebook 
    GROUP BY id_user 
) ebook 
    ON ebook.id_user = user.id_user 

与您当前查询的问题是，你最终可能会计算同一页面更如果给定的用户拥有多本书，那么这一次就是一次。按照上面的方法分开汇总是避免这种重复计算问题的一种方法。