mysql - 使用单个列对多个表进行内连接，对结果值产生 'multiplier' 影响

Question

尝试在 4 个表(项目、任务、支出和捐赠)上运行简单的内连接查询。连接似乎在某处缠结。例如，SUM(tasks.budget_amount) 应该给出 3,211,385.21 的组合值，但我得到的是 49,659,564.60

SELECT projects.id, projects.name, SUM(tasks.budget_amount) as budget, SUM(spendings.amount_spent) AS spending, SUM(donations.donation_amount) AS donation 
    FROM (((projects
    INNER JOIN tasks on tasks.project_id=projects.id)
    INNER JOIN spendings on spendings.task_id=tasks.id)
    INNER JOIN donations on donations.task_id=tasks.id)
    GROUP BY projects.id

有办法防止这种情况发生吗？

Answer 1

从 SELECT 列表 ( SUM ) 中删除聚合表达式并删除 GROUP BY ，返回每个表的主键/唯一标识符，看看发生了什么。

我们将看到的是半笛卡尔积(或叉积)，一个表中的多行与另一个表中的多行相匹配。 SQL 规范是返回所有匹配项。

<小时/>

避免这种情况的一种方法是通过预先聚合结果，然后进行连接来避免叉积。

举个例子:

SELECT p.id
     , p.name
     , IFNULL(b.budget,0)    AS budget
     , IFNULL(s.spending,0)  AS spending
     , IFNULL(d.donation,0)  AS donation 
  FROM projects p
  LEFT
  JOIN ( SELECT bt.project_id
              , SUM(bt.budget_amount) AS budget
           FROM tasks bt
         GROUP BY bt.project_id 
       ) b
    ON b.project_id = p.id
  LEFT
  JOIN ( SELECT st.project_id
              , SUM(ss.amount_spent) AS spending
           FROM tasks st
           JOIN spendings ss
             ON ss.task_id = st.id
          GROUP BY st.project_id
       ) s
    ON s.project_id = p.id
  LEFT
  JOIN ( SELECT dt.project_id
              , SUM(dd.donation_amount) AS donation
           FROM tasks dt
           JOIN donations dd
             ON dd.task_id = dt.id
          GROUP BY dt.project_id
       ) d
    ON d.project_id = p.id

请注意，每个内联 View d , s ，和b保证返回 project_id 的唯一值。