如何獲得在左另一個表的計數加入

Question

我有多個表

post 
    id Name 
    1 post-name1 
    2 post-name2 

user 
    id username 
    1 user1 
    2 user2 

post_user 
    post_id user_id 
    1   1 
    2   1 

post_comments 
    post_id comment_id 
    1   1 
    1   2 
    1   3 

我使用的是這樣的查詢：

SELECT post.id, post.title, user.id AS uid, username 
FROM `post` 
LEFT JOIN post_user ON post.id = post_user.post_id 
LEFT JOIN user ON user.id = post_user.user_id 
ORDER BY post_date DESC 

它按預期工作。不過，我希望獲得每篇文章的評論數量。那麼，我該如何修改這個查詢，以便我可以得到評論的數量。

任何想法？

Answer 1

SELECT post.id, post.title, user.id AS uid, username, COALESCE(x.cnt,0) AS comment_count 
FROM `post` 
LEFT JOIN post_user ON post.id = post_user.post_id 
LEFT JOIN user ON user.id = post_user.user_id 
LEFT OUTER JOIN (SELECT post_id, count(*) cnt FROM post_comments GROUP BY post_id) x ON post.id = x.post_id 
ORDER BY post_date DESC 

編輯：使其成爲一個外連接的情況下有沒有任何意見

EDIT2：改變ISNULL凝聚

Answer 2

這個編輯版本顯示沒有評論行：

SELECT post.id, post.title, user.id AS uid, username, count(post_comments.comment_id) as comment_count 
FROM `post` 
LEFT JOIN post_user ON post.id = post_user.post_id 
LEFT JOIN user ON user.id = post_user.user_id 
LEFT JOIN post_comments ON post_comments.post_id = post.id 
GROUP BY post.id 
ORDER BY post_date DESC 

例如：

+----+------------+------+----------+---------------+ 
| id | title  | uid | username | comment_count | 
+----+------------+------+----------+---------------+ 
| 3 | post-name3 | 2 | user2 |    0 | 
| 1 | post-name1 | 1 | user1 |    3 | 
| 2 | post-name2 | 1 | user1 |    1 | 
+----+------------+------+----------+---------------+ 
3 rows in set (0.01 sec)