Mysql 为什么评论计数错误？

我需要计算项目的评论和投票，但评论计算错误

   SELECT projects . * , COUNT( votes.project_id ) AS votes, COUNT( comments.user_id) AS comments
    FROM  `projects` 
    LEFT JOIN  `votes` ON  `projects`.`id` =  `votes`.`project_id` 
    LEFT JOIN  `comments` ON  `projects`.`id` =  `comments`.`project_id` 
    WHERE  `votes`.`created_at` >  '2014-05-31 20:21:43' AND 
    GROUP BY  `projects`.`id` 
    ORDER BY  `votes` DESC 

输出：

---

您需要计算不同的值，因此有点像：

SELECT projects . * , COUNT( DISTINCT votes.user_id ) AS votes, COUNT( DISTINCT comments.user_id) AS comments
FROM  `projects` 
LEFT JOIN  `votes` ON  `projects`.`id` =  `votes`.`project_id` 
LEFT JOIN  `comments` ON  `projects`.`id` =  `comments`.`project_id` 
WHERE  `votes`.`created_at` >  '2014-05-31 20:21:43' AND 
GROUP BY  `projects`.`id` 
ORDER BY  `votes` DESC 

计数“错误”，因为

count（）

聚合正在计算结果集中的行，而不是单个表中的行。如果在

comments

表中有两行

project_id=1

，则两个计数聚合都将返回8

基本上，

注释

中的每一行都与

项目

中的每一行相匹配

有几种方法可以解决这个问题。一种是在选择列表中使用subselect，尽管对于大型集合来说这可能会很昂贵（性能方面）：

SELECT p.*
     , ( SELECT COUNT(1)
           FROM votes v
          WHERE v.project_id = p.project_id
            AND v.created_at > '2014-05-31 20:21:43'
       ) AS votes
     , ( SELECT COUNT(1)
           FROM comments c
          WHERE c.project_id = p.project_id
       ) AS comment_cnt
  FROM projects p
HAVING votes > 0
 ORDER BY votes DESC

（包含HAVING子句是为了模拟原始查询；在原始查询中，WHERE子句中的

vows.created_at

上的谓词否定投票表左连接的“outerness”。）

---

另一种方法是分别从每个表中获取计数（在两个单独的查询中按project_id获取计数，作为内联视图引用，然后将这些计数与联接操作相结合。例如：

SELECT p.*
     , w.votes
     , IFNULL(d.comment_cnt,0) AS comment_cnt
  FROM projects p
  JOIN ( SELECT v.project_id
              , COUNT(1) AS votes
           FROM votes v
          WHERE v.created_at > '2014-05-31 20:21:43'
          GROUP BY v.project_id
       ) w
    ON w.project_id = p.project_id
  LEFT
  JOIN ( SELECT c.project_id
               , COUNT(1) AS comment_cnt
           FROM comments c
          GROUP BY c.project_id
       ) d
    ON d.project_id = p.project_id
 ORDER BY w.votes DESC

（因为该规范只返回“投票”计数大于零的行，所以我们可以使用内部联接排除没有任何“投票”的行。对于注释计数，我们使用外部联接，并简单地将任何空值替换为零

还有其他方法

性能将取决于行数、引用列的基数、可用索引、优化器选择的执行计划等