连接多个表并使用 MySQL 计数

Question

所以我把自己搞得一团糟。

基本上，我有一个社交网络应用程序，我有 4 个不同的表格需要合并到一个视图中。

Table 1 - 用户 posts
Table 2 - 用户
Table 3 - 赞
Table 4 - 评论

然后我需要 return 一个包含用户详细信息的 post 列表，然后分别为每个 post 添加点赞数和评论数列。

如果 post 没有任何喜欢或评论，那么理想情况下我们应该显示零。

下面的查询将所有内容连接起来，但是 return 包含多行所有内容，因为它 return 为每个评论或类似内容添加 1 行。

谁能帮我把这些结合起来？

SELECT *
FROM app_posts AS p
LEFT JOIN app_comments AS c ON c.post_id = p.post_id
LEFT JOIN app_user AS u ON u.user_id = p.user_id
LEFT JOIN app_likes AS l ON l.post_id = p.post_id
WHERE u.user_banned = 0
AND p.post_public = 1
ORDER BY p.post_date DESC

如有任何帮助，我们将不胜感激！

Table列如下；

app_likes

like_id
post_id
user_id
liked_date

app_comments

comment_id
评论_user_id
post_id
comment_body
comment_date

app_posts

post_id
user_id
post_content
post_date
post_public

app_user

user_id
user_first
user_last
user_avatar
user_banned

当前 returned 的示例如下（为方便起见，已删减）

您会看到 post_id 重复了多次。

我想要的 return 只是 post_id 一次，并且在新列中包含 'likes' 和 'comments' 的计数（我不知道如何做到这一点）。

西蒙

Answer 1

您可能缺少 GROUP BY...

SELECT p.*,u.*,count(distinct c.comment_id),count(distinct l.like_id)
FROM app_posts AS p
LEFT JOIN app_comments AS c ON c.post_id = p.post_id
LEFT JOIN app_user AS u ON u.user_id = p.user_id
LEFT JOIN app_likes AS l ON l.post_id = p.post_id
WHERE u.user_banned = 0
AND p.post_public = 1
GROUP BY p.post_id
ORDER BY p.post_date DESC

请注意，MySQL 让您可以像这样草率地使用 GROUP BY，但是很多其他数据库 would require 您可以将“p.*”分解为显式 MAX(p.post_id),MAX(p.post_content),等

Answer 2

SELECT p.post_id, COUNT(c.post_id) as num_comments, COUNT(l.like_id) as num_likes
FROM app_posts AS p
LEFT JOIN app_comments AS c ON c.post_id = p.post_id
LEFT JOIN app_user AS u ON u.user_id = p.user_id
LEFT JOIN app_likes AS l ON l.post_id = p.post_id
WHERE u.user_banned = 0
AND p.post_public = 1
GROUP BY  p.post_id
ORDER BY p.post_date DESC

Answer 3

尝试在查询末尾添加分组依据，如下所示

SELECT *
FROM app_posts AS p
LEFT JOIN app_comments AS c ON c.post_id = p.post_id
LEFT JOIN app_user AS u ON u.user_id = p.user_id
LEFT JOIN app_likes AS l ON l.post_id = p.post_id
WHERE u.user_banned = 0
AND p.post_public = 1
GROUP BY p.post_id
ORDER BY p.post_date DESC

连接多个表并使用 MySQL 计数

Joining multiple Tables and using Count with MySQL

mysql

join

inner-join

left-join