Question

我正在建立一个讨论区......我有一个包含帖子的表和另一个包含评论的表...外国id是post_id。现在，我正在尝试根据每个评论的数量对帖子进行排序。我正在使用的查询：

SELECT username, posts.post_id, category_id, UNIX_TIMESTAMP(posts.datetime) AS datetime, posts.body, posts.owner_id, COUNT(comment_id) AS number
FROM posts, comments, user
WHERE posts.post_id = comments.post_id AND posts.owner_id = user.`id`
GROUP BY comments.post_id
ORDER BY number DESC

查询工作正常，但问题是没有选择没有评论的帖子。如果他们有评论，我想显示帖子而不是他们，但我想按评论数量对它们进行排序。

Answer 1

使用LEFT加入。你的From / where关节相当于INNER JOIN：

SELECT username, p.post_id, category_id, UNIX_TIMESTAMP(p.datetime) AS datetime,
   p.body, p.owner_id, COUNT(comment_id) AS number
FROM posts p
LEFT JOIN comments c ON p.post_id = c.post_id
LEFT JOIN user u ON p.owner_id = u.`id`
GROUP BY c.post_id
ORDER BY number DESC

Answer 2

在评论表 LEFT JOIN

试试这个：

SELECT username, p.post_id, category_id, UNIX_TIMESTAMP(p.datetime) AS DATETIME, 
       p.body, p.owner_id, COUNT(comment_id) AS number
FROM posts p 
INNER JOIN user u ON p.owner_id = u.id
LEFT JOIN comments c ON p.post_id = c.post_id 
GROUP BY p.post_id
ORDER BY number DESC;

当另一个表上的行计数为0时，没有得到结果

2 个答案: