从另一个表计数的SQL

Question

我正在管理用户可以喜欢的社交网络的数据库，发表评论，并且我为每篇文章，例如和评论创建了一个表格。

like --- post_id 
     user_id 

comment --- post_id 
      user_id 

现在，当我运行select * from post，我希望能够增加两列likes和comments计数喜欢和有每种post_id评论的数量。

我该怎么做？

Answer 1

您需要加入到这两个表中。

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 

Answer 2

SELECT 
    post.*, 
    IFNULL(COUNT(like.user_id),0) AS likecount, 
    IFNULL(COUNT(comment.user_id),0) AS commentcount 
FROM 
    post 
    LEFT JOIN like ON post.post_id=like.post_id 
    LEFT JOIN comment ON post.post_id=comment.post_id 
GROUP BY 
    like.post_id 

Answer 3

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 

Demo on sqlfiddle.

Answer 4

​​

这就像@Richard的答案，但修改select，因为在group by列必须是select子句。