Mysql 从另一个表计数的SQL

我正在为一个社交网络管理一个数据库，在那里用户可以喜欢、评论，我为每个帖子、喜欢和评论创建了一个表

like --- post_id
         user_id

comment --- post_id
            user_id

现在，当我运行

select*from post

时，我希望能够添加两列

likes

和

comments

，计算每个

post\u id

的like和comments的数量

---

我如何才能做到这一点？

您将需要连接到两个表

SELECT post.*, COUNT(likes.id) AS like_count, COUNT(comment.id) AS comment_count
FROM post
    LEFT JOIN likes ON post.id = likes.post_id
    LEFT JOIN comment ON post.id = comment.post_id
GROUP BY post.id

像这样：

SELECT *
,    (SELECT COUNT(*) FROM likes WHERE post_id=p.id) as LIKE_COUNT
,    (SELECT COUNT(*) FROM comment WHERE post_id=p.id) as COMMENT_COUNT
FROM post p

---

这就像修改

select

的答案一样，因为

groupby

中的列必须在

select

子句中。

在SQL Server下，我同意-但是，MySQL对GROUPBY中的所有select字段的命名没有那么严格。你可以按主键分组，但仍然可以从表中选择*哦，我不知道。评论不错。

SELECT *
,    (SELECT COUNT(*) FROM likes WHERE post_id=p.id) as LIKE_COUNT
,    (SELECT COUNT(*) FROM comment WHERE post_id=p.id) as COMMENT_COUNT
FROM post p

SELECT post.id, COUNT(like.id) AS like_count, COUNT(comment.id) AS comment_count
FROM post p
    LEFT JOIN like ON post.id = like.post_id
    LEFT JOIN comment ON post.id = comment.post_id
GROUP BY post.id