SQL多个COUNT（）从两个表，在LEFT JOIN [英] SQL Multiple COUNT() from two tables, within a LEFT JOIN

问题描述

我有三个表，我想从其中一个表中选择所有数据，并且还收集其他两个表中链接特定行数据的次数COUNT。

i have three tables, i want to select all data from one of the tables, and also collect a COUNT of how many times that specific row of data is linked in the other two tables.

所以，从site_projects中选择所有数据。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

So, SELECT all data from site_projects. Then return a COUNT of site_project_members WHERE site_projects.id = site_project_members.pid and also return a COUNT of site_project_tasks WHERE site_projects.id = site_project_members.pid

我有意义，对查询，它看起来正确。并且它查询数据库（MySQL）没有问题。 除之外，它会同时返回两个计数的总和。 （见下表结构）

I hope i'm making sense, to the query, it looks correct. and it querys the database (MySQL) with no problems. Except it returns a sum of both of the counts as both things. (see below the table structures)

site_projects

site_projects

id | title     | desc      | start      | deadline   | progress

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

site_project_members

site_project_members

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

site_project_tasks

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

这两个值都应该为2.然而它们不是：S任何人都能帮帮我？
非常感谢

Both of the "4" values should be 2. however they are not :S Anyone able to help? Much Appreciated

感谢，
Dan

Thanks, Dan

推荐答案

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

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