mysql - 奇怪的MySQL用聚合函数连接查询结果

Question

我编写了以下join 查询以使用聚合 函数获取报告

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

但我不知道为什么会得到以下输出？

聚合函数的结果相互相乘!!!

我不知道为什么？

例如，对于最后一行，我希望得到以下结果

821    |    48000    |    63    |    0    |    10    |    10

执行EXPLAIN查询的结果如下所示

Answer 1

出现这种类型结果的一个原因是您正在对用户表使用左连接，结果集可能包含每个用户的重复行，因此您得到的计数比预期的多，您可以使用 DISTINCT 在计数中仅计算每个用户的唯一关联，对于 totalCost 的总和，您可以使用子选择为每个用户计算总和，而无需为用户订单重复值

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