從另一個表計數的SQL

Question

我正在管理用戶可以喜歡的社交網絡的數據庫，發表評論，並且我爲每篇文章，例如和評論創建了一個表格。

like --- post_id 
     user_id 

comment --- post_id 
      user_id 

現在，當我運行select * from post，我希望能夠增加兩列likes和comments計數喜歡和有每種post_id評論的數量。

我該怎麼做？

Answer 1

您需要加入到這兩個表中。

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

像這樣：

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

​​

這就像@Richard的答案，但修改select，因爲在group by列必須是select子句。