奇怪的MySQL连接查询的结果与聚合函数

Question

我写了下面加入查询来获取使用聚合功能

SELECT users.id, SUM(orders.totalCost) AS bought, COUNT(comment.id) AS commentsCount, COUNT(topics.id) AS topicsCount, COUNT(users_login.id) AS loginCount, COUNT(users_download.id) AS downloadsCount 
FROM users 
LEFT JOIN orders ON users.id=orders.userID AND orders.payStatus=1 
LEFT JOIN comment ON users.id=comment.userID 
LEFT JOIN topics ON users.id=topics.userID 
LEFT JOIN users_login ON users.id=users_login.userID 
LEFT JOIN users_download ON users.id=users_download.userID 
GROUP BY users.id 
ORDER BY bought DESC 

，但我不知道为什么我得到下面的输出报告？

聚合函数的结果是相互相乘的！

我不知道为什么？

例如用于最后行i预期以下结果

821 | 48000 | 63 | 0 | 10 | 10 

执行EXPLAIN查询的结果示于下

Answer 1

原因之一用于该类型的结果将是您正在使用用户表的左连接，并且结果集可能包含dup每个用户的授权行，所以你得到的数量超过预期的数量，你可以在count中使用DISTINCT来计算每个用户的唯一关联，并且总和为totalCost，你可以使用一个子查询为每个用户获得总和，而不必对于用户订单

SELECT 
    u.id, 
    COALESCE(o.bought,0) bought 
    COUNT(DISTINCT c.id) AS commentsCount, 
    COUNT(DISTINCT t.id) AS topicsCount, 
    COUNT(DISTINCT ul.id) AS loginCount, 
    COUNT(DISTINCT ud.id) AS downloadsCount 
FROM users u 
LEFT JOIN (SELECT 
      userID, 
      SUM(totalCost) bought 
      FROM orders 
      WHERE payStatus=1 
      GROUP BY userID) o 
ON u.id=o.userID 
LEFT JOIN `comment` c ON u.id=c.userID 
LEFT JOIN topics t ON u.id=t.userID 
LEFT JOIN users_login ul ON u.id=ul.userID 
LEFT JOIN users_download ud ON u.id=ud.userID 
GROUP BY u.id 
ORDER BY bought DESC