Question

我有一张posts表。我通过以下方式加入votes表：

SELECT posts.*,
       (CASE
            WHEN vs.user_id = 1 THEN 0
            ELSE 1
        END) AS voted_by_me
FROM `posts`
LEFT OUTER JOIN votes AS vs ON vs.post_id = posts.id
GROUP BY posts.id

基本上我想知道连接值是否与user_id = 1.这显然不起作用，因为它只会在CASE WHEN条件中使用随机返回值。我想知道是否有办法找出连接值group by是否包含特定值。

澄清： 我想获取所有帖子，并且每个帖子都有一个名为voted_by_me的列，如果帖子由id为1的用户投票，则将其设为0，否则为1.

Answer 1

这就是你想要的吗？

SELECT posts.id,
       sum(vs.user_id = 1 AND vs.option = 0) > 0 AS downvoted_by_me,
       sum(vs.user_id = 1 AND vs.option = 1) > 0 AS upvoted_by_me
FROM `posts`
LEFT OUTER JOIN votes AS vs ON vs.post_id = posts.id
GROUP BY posts.id

Answer 2

SELECT posts.id,
       ifnull(vs.isvoted, 0) AS voted_by_me
FROM `posts`
LEFT OUTER JOIN (select distinct postid , 1 as isvoted from votes where user_id=1) AS vs
ON vs.post_id = posts.id

使用JOIN / GROUP BY时的MySQL案例

2 个答案: