Question

我的SQL查询包含posts表，post_likes表和comments表的三个表。所有表都与posts表中的post_id主键相关联。我试图返回帖子行的内容以及它在post_likes表中的喜欢/不喜欢的数量，以及评论的数量。查询工作正常，直到我引入了第二个左连接，它现在显示了like_count列x5 dislike_count列x5和新的comment_count x4。

这是有问题的查询：

SELECT c.post_id, c.post_name, c.post_content, c.post_datetime, c.user_name, sum(p.like_count) AS like_count, sum(p.dislike_count) AS dislike_count, sum(s.comment_count) AS comment_count FROM posts c LEFT JOIN post_likes p ON c.post_id = p.post_id LEFT JOIN comments s ON c.post_id = s.post_id WHERE c.user_name = 'test' GROUP BY c.post_id

返回总和值：

//column | returned value | expected value

    like_count | 10 | 2
    dislike_count | 5 | 1
    comment_count | 20 | 5

一些额外的注释，喜欢/不喜欢的内容存储在带有结构的postlikes表中。

post_like_id, like_count, dislike_count, post_id, user_name

在PHP处理此列的任一列中，喜欢或不喜欢的计数只能为1，以确保用户不能多次等，而user_name列是喜欢该帖子的用户。

评论表结构如下：

comment_id, comment_name, comment_content, comment_datetime, comment_count, post_id, user_name

插入时，comment_count始终为1以允许sum函数，post_id是注释的帖子的id，而user_name是评论的用户。

Answer 1

您的joins正在生成笛卡尔积 - 而是将聚合结果移到子查询中：

SELECT c.post_id, c.post_name, c.post_content, c.post_datetime, c.user_name, 
    p.like_count, 
    p.dislike_count, 
    s.comment_count
FROM posts c 
    LEFT JOIN (
       select post_id, 
              sum(like_count) like_count,
              sum(dislike_count) dislike_count
       from post_likes
       group by post_id
    ) p ON c.post_id = p.post_id 
    LEFT JOIN (
       select post_id, sum(comment_count) comment_count
       from comments
       group by post_id
    ) s ON c.post_id = s.post_id 
WHERE c.user_name = 'test'

SQL查询返回Sum和Count的Wring值

1 个答案: