来自两个表的SQL多个COUNT(),在LEFT JOIN中

发布时间：2020-12-12 08:34:11 所属栏目：MsSql教程来源：网络整理

导读：我有三个表,我想从其中一个表中选择所有数据,并且还会收集在其他两个表中链接特定数据行的次数的COUNT. 所以,从site_projects中选择所有的数据.然后返回一个COUNT的site_project_members WHERE site_projects.id = site_project_members.pid,并返回一个COUNT

我有三个表,我想从其中一个表中选择所有数据,并且还会收集在其他两个表中链接特定数据行的次数的COUNT.

所以,从site_projects中选择所有的数据.然后返回一个COUNT的site_project_members WHERE site_projects.id = site_project_members.pid,并返回一个COUNT的site_project_tasks WHERE site_projects.id = site_project_members.pid

我希望我有意义,查询,看起来正确.它会查询数据库(MySQL),没有问题.除了它返回两个计数的总和作为两件事情. (见下表结构)

site_projects

id | title     | desc      | start      | deadline   | progress

1  | Project 1 | a project | 1321748906 | 1329847200 | 20

site_project_members

id | pid | uid | img | hidden
1  | 1   | 1   | 1   | 0
2  | 1   | 2   | 2   | 0

site_project_tasks

id | pid | desc   | completed
1  | 1   | Task 1 | 1
1  | 1   | Task 2 | 0

这是我的查询：

SELECT p.`id`,p.`title`,p.`desc`,p.`progress`,p.`start`,p.`deadline`,COUNT(m.`id`) as `members`,COUNT(t.`id`) as `tasks` FROM `site_projects` p LEFT JOIN `site_project_members` m ON p.`id`=m.`pid` LEFT OUTER JOIN `site_project_tasks` t ON p.`id`=t.`pid` ORDER BY p.`id` ASC

我得到的结果是：

id | title     | desc      | progress | start      | deadline   | members | tasks
1  | Project 1 | a project | 20       | 1321748906 | 1329847200 | 4       | 4

两个“4”值都应该是2.但是它们不是：S任何人都可以帮忙？
非常感激

谢谢,
担

解决方法

SELECT p.id,p.title,p.desc,p.progress,p.start,p.deadline,COALESCE( m.cnt,0 ) AS members,COALESCE( t.cnt,0 ) AS tasks
FROM site_projects p
LEFT JOIN 
  ( SELECT pid,COUNT(*) AS cnt FROM
    site_project_members
    GROUP BY pid ) m
ON p.id = m.pid
LEFT JOIN
  ( SELECT pid,COUNT(*) AS cnt FROM
    site_project_tasks
    GROUP BY pid ) t
ON p.id = t.pid
ORDER BY p.id ASC

（编辑：李大同）

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