MySQL:在执行多个联接时如何求和非重复值

Question

我尝试从包含一些连接的查询中对列中的值进行求和。

示例：

SELECT
    p.id AS product_id,
    SUM(out_details.out_details_quantity) AS stock_bought_last_month,
    SUM(order_details.order_quantity) AS stock_already_commanded
FROM product AS p 
INNER JOIN out_details ON out_details.product_id=p.id 
INNER JOIN order_details ON order_details.product_id=p.id 
WHERE p.id=9507
GROUP BY out_details.out_details_pk, order_details.id;

我得到的结果是：

+------------+-------------------------+-------------------------+
| product_id | stock_bought_last_month | stock_already_commanded |
+------------+-------------------------+-------------------------+
|       9507 |                      22 |                      15 |
|       9507 |                      22 |                      10 |
|       9507 |                      10 |                      15 |
|       9507 |                      10 |                      10 |
|       9507 |                       5 |                      15 |
|       9507 |                       5 |                      10 |
+------------+-------------------------+-------------------------+

现在，我想对这些值求和，但当然有重复的值。我还必须按product_id分组：

SELECT 
  p.id AS product_id,
  SUM(out_details.out_details_quantity) AS stock_bought_last_month,
  SUM(order_details.order_quantity) AS stock_already_commanded
FROM product AS p 
INNER JOIN out_details ON out_details.product_id=p.id 
INNER JOIN order_details ON order_details.product_id=p.id 
WHERE p.id=9507
GROUP BY p.id;

结果：

+------------+-------------------------+-------------------------+
| product_id | stock_bought_last_month | stock_already_commanded |
+------------+-------------------------+-------------------------+
|       9507 |                      74 |                      75 |
+------------+-------------------------+-------------------------+

想要的结果是：

+------------+-------------------------+-------------------------+
| product_id | stock_bought_last_month | stock_already_commanded |
+------------+-------------------------+-------------------------+
|       9507 |                      37 |                      25 |
+------------+-------------------------+-------------------------+

如何忽略重复项？当然，行数可以改变！

Answer 1

Select P.Id
    , Coalesce(DetailTotals.Total,0) As stock_bought_last_month
    , Coalesce(OrderTotals.Total,0) As stock_already_commanded
From product As P
    Left Join   (
                Select O1.product_id, Sum(O1.out_details_quantity) As Total
                From out_details As O1
                Group By O1.product_id
                ) As DetailTotals
        On DetailTotals.product_id = P.id
    Left Join   (
                Select O2.product_id, Sum(O2.order_quantity) As Total
                From order_details As O2
                Group By O2.product_id
                ) As OrderTotals
        On OrderTotals.product_id = P.id
Where P.Id = 9507   

Answer 2

另一种方法：

SELECT
    p.product_id,
    p.stock_bought_last_month,
    SUM(order_details.order_quantity) AS stock_already_commanded
from
(SELECT
    product.id AS product_id,
    SUM(out_details.out_details_quantity) AS stock_bought_last_month,
FROM product 
INNER JOIN out_details ON out_details.product_id=product.id 
WHERE product.id=9507
group by product.id
) AS p 
INNER JOIN order_details ON order_details.product_id=p.product_id
group by p.product_id;

严格地说，group by子句在本例中是不必要的，因为只有一个产品id -但是，如果选择了多个产品id，则它们将是必需的。