MySQL查询与多个INNER JOIN

Question

我有点困惑的一个愚蠢的查询：

我得到行从表帖子加入了与表作者和表评论，在这样的方式：

SELECT posts.*, authors.name, COUNT(comments.id_post) AS num_comments 
FROM posts JOIN authors ON posts.id_author = authors.id_author 
LEFT JOIN comments ON posts.id_post = comments.id_post 
WHERE posts.active = 1 
AND comments.active = 1 

这当然不起作用。

我尝试做的是检索：

1）我的所有积极的职位（如那些删除未标记）;

2）他们作者的名字;

3）每个帖子（如果至少有一个）的活动评论数（没有标记为删除的评论数）;

这是怎么回事？我知道这是一个微不足道的，但现在我的大脑处于越位...... 谢谢！

Answer 1

既然你正在做一个计数，你需要有一个组。因此您需要添加

Group By posts.*, authors.name 

Answer 2

您应该将GROUP BY子句与聚合函数一起使用。尝试类似于：

SELECT posts.*, authors.name, COUNT(comments.id_post) AS num_comments 
FROM posts JOIN authors ON posts.id_author = authors.id_author 
LEFT JOIN comments ON posts.id_post = comments.id_post 
-- group by 
GROUP BY posts.*, authors.name 
-- 
WHERE posts.active = 1 
AND comments.active = 1 

Answer 3

据推测，id_post唯一标识posts每一行。试试这个：

SELECT p.*, a.name, COUNT(c.id_post) AS num_comments 
FROM posts p JOIN 
    authors a 
    ON p.id_author = a.id_author LEFT JOIN 
    comments c 
    ON p.id_post = c.id_post 
WHERE p.active = 1 AND c.active = 1 
GROUP BY p.id_post; 

请注意，这使用MySQL扩展。在大多数其他数据库中，您需要列出posts加上子句中的a.name的所有列。

编辑：

以上是根​​据您的查询。如果你想用积极的意见计数所有活动的帖子，只是做：

SELECT p.*, a.name, SUM(c.active = 1) AS num_comments 
FROM posts p LEFT JOIN 
    authors a 
    ON p.id_author = a.id_author LEFT JOIN 
    comments c 
    ON p.id_post = c.id_post 
WHERE p.active = 1 
GROUP BY p.id_post; 

Answer 4

我找到了正确的解决方案：

SELECT posts.id_post, authors.name, COUNT(comments.id_post) AS num_comments 
FROM posts JOIN authors 
ON posts.id_author = authors.id_author 
LEFT OUTER JOIN comments 
ON (posts.id_post = comments.id_post AND comments.active = 1) 
WHERE posts.active = 1 
GROUP BY posts.id_post; 

感谢大家的帮助！