SQL Multiple COUNT() 来自两个表，在一个 LEFT JOIN 中

Question

我有三个表，我想从其中一个表中选择所有数据，并收集该特定数据行在其他两个表中链接的次数的 COUNT。

所以，从 site_projects 中选择所有数据。然后返回 COUNT of site_project_members WHERE site_projects.id = site_project_members.pid 并返回 COUNT of site_project_tasks WHERE site_projects.id = site_project_members.pid

我希望我是有道理的，对于查询，它看起来是正确的。它查询数据库（MySQL）没有问题。 除了它返回两个计数的总和作为两个东西。 （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

这两个“4”值都应该是 2。但是它们不是 :S 有人能帮忙吗？ 非常感谢

谢谢， 丹

Answer 1

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

Answer 2

SELECT p.`id`, p.`title`, p.`desc`, p.`progress`, p.`start`, p.`deadline`,
     (SELECT COUNT(*) FROM site_project_members m WHERE p.`id`=m.`pid`) AS `members`,
     (SELECT COUNT(*) FROM site_project_tasks t WHERE p.`id`=t.`pid`) AS `tasks`
FROM `site_projects` p
ORDER BY p.`id` ASC

这确实可以解决问题。