MySQL：如何从表中选择两个字段，并在两个相关表中选择合计列？

Question

我有三个表：

Project PurchaseA PurchaseB

项目架构：

id | user_id | title | description | create_date

购买方案A：

id | project_id | amount | create_date

PurchaseB模式：

id | project_id | amount | create_date

Project有一堆独特的行，详细说明了不同的项目。 这些项目中的每个项目在PurchaseA和PurchaseB都有很多行。 我想要做的是选择所有从行的Project与领域的id和title ，也都总在相关记录的总和PurchaseA和总的相关记录PurchaseB 。

我可以使它彼此独立地工作，但是当我有两个联接时，数字最终会大大膨胀。

所需结果

21732 Test Project A          84274.48
21747 Test Project B          18237.47
34714 Test Project C         183747.18

其中带有FLOATS的列是PurchaseA和PurchaseB相关行的总和。

编辑

这是我当前的SQL：

select 
    projects.id,
    projects.title,
    COALESCE(SUM(PurchaseA.amount), 0) as pledgetotal,
    COALESCE(SUM(PurchaseB.amount), 0) as purchasetotal
from
    projects
        left join
    PurchaseA ON (PurchaseA.project_id = projects.id)
        left join
    PurchaseB ON (PurchaseB.project_id = projects.id)
        left join

WHERE
    projects.title IS NOT NULL

    projects.end_date < NOW()
group by projects.id

测试数据

Projects:

id          title           description         create_date

623         Test Project A  This is a test A    2013-01-01
624         Test Project B  This is a test B    2013-01-02


PurchaseA

id          project_id      amount              create_date
1           623             100                 2013-01-02
2           623             125                 2013-01-02
3           624             200                 2013-01-03
4           623             150                 2013-01-03
5           624             50                  2013-01-04

PurchaseB

id          project_id      amount              create_date
1           623             110                 2013-01-02
2           624             105                 2013-01-02
3           623             215                 2013-01-03
4           623             100                 2013-01-03
5           624             150                 2013-01-04

如您所见，每个项目在每个相关表中都有多个行。 我想从每个表中获取总和，然后将它们加在一起。

Answer 1

我相信您的问题的部分原因在于projects与purchase.之间存在一对多的关系purchase. 表。 我的建议是将sum()移到子查询：

select 
    p.id,
    p.title,
    a.pledgetotal,
    b.purchasetotal
from projects p
left join
(
  select project_id, sum(amount) pledgetotal
  from PurchaseA
  group by project_id
) a
  ON (a.project_id = p.id)
left join
( 
  select project_id, sum(amount) purchasetotal
  from PurchaseB
  group by project_id
) b
  ON (B.project_id = p.id)
WHERE p.title IS NOT NULL;

参见带有演示的SQL Fiddle

Answer 2

您可以尝试以下操作（未经测试）：

SELECT a.id, a.title, (pa.amount+pb.amount)  AS TotalPurchase FROM Project a
    LEFT JOIN ( SELECT project_id, SUM(amount) AS amount FROM PurchaseA GROUP BY project_id ) pa
    ON pa.project_id = a.id
    LEFT JOIN ( SELECT project_id, SUM(amount) AS amount FROM PurchaseB GROUP BY project_id ) pb
    ON pb.project_id = a.id