strange MySQL join query results with aggregate functions - mysql

Question

i wrote the following join query to get a report using aggregate functions
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
but i don't know why i get the following output?
the result of aggregate functions are multiplied with each other!!!
i don't know why?
for example for the last row i expected the following result
821 | 48000 | 63 | 0 | 10 | 10
the result of executing EXPLAIN query are shown below

Answer 1

One reason for that type of result would be you are using a left joins with your users table and the result set may contains the duplicate rows for each user so you are getting count more than the expected one for this you can use DISTINCT in count to count only the unique associations per user and for sum of totalCost you can use a subselect to have a sum for each user without having repeated values for orders of user
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