MySQL查询多个LEFT加入问题

Question

我试图从以下查询获得所需的结果，但它似乎没有工作...

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

如果我删除最后2个LEFT jOINS，查询将起作用...其他2的错误是什么？ 我是否必须使用其他方法才能工作？

谢谢

Answer 1

我想通过'不工作'你的意思是查询返回太多记录？ 这是因为联接的组合。 您将返回每篇文章记录的每个回复，因此数字会相乘。 您可以通过在COUNT中使用DISTINCT来解决此问题。 这样，您可以计算唯一ID，因此您只需计算一次每篇文章：

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

[编辑]

一个可能更快的解决方案，无需DISTINCT和GROUP BY：

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