Question

我对一个愚蠢的查询有点困惑：

我从表作者和表格评论加入表帖子的行，方式如下：

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

当然，这不起作用。

我尝试做的是检索：

1）我所有的活动帖子（未标记为已删除的帖子）;

2）作者姓名;

3）每个帖子的活动评论数量（未标记为删除的评论数量）（如果至少有一个）;

有什么办法？我知道这是一个微不足道的，但到现在为止我的大脑越位了...... 谢谢！

Answer 1

据推测，id_post唯一标识posts中的每一行。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，您需要在posts子句中列出a.name加group by中的所有列。

编辑：

以上内容基于您的查询。如果您希望所有活动帖子都有活跃评论，请执行以下操作：

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 2

由于你正在计算，你需要有一个分组。所以你需要添加

Group By posts.*, authors.name

Answer 3

你应该将GROUP BY子句与聚合函数一起使用。尝试类似的事情：

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 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！

具有多个INNER JOIN的MySQL查询

4 个答案: