MySQL查詢多個LEFT聯接問題

Question

我想從以下查詢所需的結果，但它似乎沒有工作...

SELECT DISTINCT 
u.user_name as user_name, 
u.total_points as total_points, 
u.user_id as user_id, 
COUNT(a.id) as user_total_articles_published, 
COUNT(r.id) as user_total_replies_published, 
COUNT(v.id) as user_total_votes_done 
FROM users as u 
LEFT JOIN articles as a ON u.user_id=a.user_id 
LEFT JOIN replies as r ON u.user_id=r.user_id 
LEFT JOIN votes as v ON u.user_id=v.user_id 
GROUP BY u.user_id 
ORDER BY u.total_points DESC 
LIMIT 10 

如果我刪除最後2 LEFT JOIN的查詢將正常工作。 ..其他2什麼錯了？我是否必須使用其他方法才能正常工作？

感謝

Answer 1

我認爲「不工作」你的意思是，查詢返回太多記錄？這是因爲聯合的組合。您返回每篇文章記錄的每個回覆，以便數字相乘。您可以通過在COUNT中使用DISTINCT來解決此問題。這樣一來，你算的唯一ID的，所以你指望每篇文章只有一次：

COUNT(distinct a.id) as user_total_articles_published, 
COUNT(distinct r.id) as user_total_replies_published, 
COUNT(distinct v.id) as user_total_votes_done 

[編輯]

一個可能更快的解決方案，省去了DISTINCT和GROUP BY的需要：

SELECT 
    u.user_name as user_name, 
    u.total_points as total_points, 
    u.user_id as user_id, 
    (SELECT COUNT(a.id) FROM articles a 
    WHERE a.user_id = u.user_id) as user_total_articles_published, 
    (SELECT COUNT(r.id) FROM replies r 
    WHERE r.user_id = u.user_id) as user_total_replies_published, 
    (SELECT COUNT(v.id) FROM votes v 
    WHERE v.user_id = u.user_id) as user_total_votes_done 
FROM users as u 
ORDER BY u.total_points DESC 
LIMIT 10