Question

现在已经坚持使用这个相当简单的MySQL查询了一天！简直不敢相信我忘记了基础知识。我有3个表 - user，post和favourite_post。 post表的user_id是FK到用户表。
favourite_post表有

user_id REFERENCES user(id) 
post_id REFERENCES post(id)
timestamp

当用户收到帖子时，他的user_id, post_id和timestamp会被插入到favourite_post表中。

我使用以下查询来检索最近的15篇帖子

SELECT post.id, post.text, post.timestamp, post.user_id, user.username 
FROM post 
   INNER JOIN user 
   ON post.user_id = user.id 
ORDER BY post.id 
DESC LIMIT 15;

我需要做的就是通过加入favourite_post表来检查每个帖子是否已被当前用户（比如user_id = 1）所青睐。

Answer 1

    SELECT p.id, p.text, p.timestamp, p.user_id, u.username, 
           IF(ISNULL(fp.post_id), 'No', 'Yes') has_favourite
      FROM post p
INNER JOIN user u
        ON p.user_id = u.id 
 LEFT JOIN favourite_post fp
        ON p.id = fp.post_id
       AND u.id = fp.user_id
  ORDER BY p.id DESC 
     LIMIT 15;

Answer 2

试试这个：

SELECT p.id, p.text, fp.timestamp, p.user_id, u.username 
FROM post p INNER JOIN user u
    ON p.user_id = u.id
LEFT JOIN favourite_post fp
    ON fp.user_id = p.user_id AND fp.post_id = p.id
WHERE p.user_id = your_user_id
ORDER BY p.id DESC 
LIMIT 15;

一个简单的MySQL查询帮助

2 个答案: