MySQL 左连接计数活动并返回 null

Question

试图让这个mysql 查询工作。

解释：有两张表，一张包含所有带有 org id 列的项目，第二张包含与项目表相关的列 projectid 的所有活动。我想计算活动表中具有用户 ID 并且活动为 1 的所有活动。 但棘手的部分来了，我无法工作。它不会收集活动表中缺少的项目，并以计数 0 显示这些项目。我想显示与 bo 中唯一 projectid 对应的所有项目

SELECT p1.`id` AS projectid ,count(a1.`id`) AS activitiescount ,a1.`userid` AS userid ,p1.`orgid` AS orgid  
FROM `projects` AS p1 
LEFT JOIN `activities` AS a1 
ON p1.`id`=a1.`projectid` 
WHERE p1.`orgid`=3 
AND a1.`userid` = 26 
AND a1.`active`=1 
OR a1.`id` IS null 
GROUP BY p1.`id` 
ORDER BY p1.`name` DESC

给予：

如您所见，例如 6 和 2 是不正确的。 请帮忙。谢谢。

Answer 1

未经测试：

将活动限制移至加入。

此外，通过移动 where 子句来解决该问题，OR 带来的结果也超出了您的预期。

标准方法：

SELECT p1.`id` AS projectid 
     , count(a1.`id`) AS activitiescount 
     , a1.`userid` AS userid 
     , p1.`orgid` AS orgid  
FROM `projects` AS p1 
LEFT JOIN `activities` AS a1 
  ON p1.`id`=a1.`projectid` 
 AND a1.`userid` = 26 
 AND a1.`active`=1 
WHERE p1.`orgid`=3 
GROUP BY p1.`id` 
ORDER BY p1.`name` DESC

您的方法 w/ () 放置正确。

SELECT 
    p1.`id` AS projectid 
    ,count(a1.`id`) AS activitiescount 
    ,a1.`userid` AS userid 
    ,p1.`orgid` AS orgid  

FROM `projects` AS p1 
LEFT JOIN `activities` AS a1 ON p1.`id`=a1.`projectid` 

WHERE p1.`orgid`=3 
  AND ((a1.`userid` = 26 AND a1.`active`=1)  OR a1.`id` IS null) 
GROUP BY p1.`id` 
ORDER BY p1.`name` DESC

----更新以模拟每个评论的完全外部连接。

SELECT * 
FROM (
SELECT p1.`id` AS projectid 
     , count(a1.`id`) AS activitiescount 
     , a1.`userid` AS userid 
     , p1.`orgid` AS orgid  
FROM `projects` AS p1 
LEFT JOIN `activities` AS a1 
  ON p1.`id`=a1.`projectid` 
 AND a1.`userid` = 26 
 AND a1.`active`=1 
WHERE p1.`orgid`=3 
GROUP BY p1.`id` 
UNION  
SELECT p1.`id` AS projectid 
     , count(a1.`id`) AS activitiescount 
     , a1.`userid` AS userid 
     , p1.`orgid` AS orgid  
FROM `projects` AS p1 
RIGHT JOIN `activities` AS a1 
  ON p1.`id`=a1.`projectid` 
 AND p1.`orgid`=3 
WHERE a1.`userid` = 26 
 AND a1.`active`=1 
GROUP BY p1.`id`)
ORDER BY p1.`id`,A1.ID -- assumes A1.ID exists and you want to order in that fashion.