MySQL：如何在进行多个 JOINS 时对非重复值求和答案

【问题标题】：MySQL: How do I SUM non duplicates values when doing multiples JOINSMySQL：如何在进行多个 JOINS 时对非重复值求和
【发布时间】：2022-01-03 13:01:47
【问题描述】：

我尝试从包含一些 JOINS 的查询中对列中的值求和。

例子：

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;

我得到这个结果：

+------------+-------------------------+-------------------------+
| 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，但当然有重复项。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！

【问题讨论】：

我认为你将不得不做两个子查询。一个在 product_id 和 stock_bought_last_month 上分组，另一个在 product_id 和 stock_already_commanded 上分组。每个值将 sum(stock) 除以 count(stock) 并使用 where 过滤掉零个计数。
为什么要按 out_details.out_details_pk 分组？
这能回答你的问题吗？ sql sum data from multiple tables

标签： sql algorithm syntax mysql

【解决方案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

【讨论】：

就是这样！我不知道我可以在 JOIN 项目中有子查询。谢谢你，这真的很有帮助！

【解决方案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;

严格来说，在这个例子中 group by 子句是不必要的，因为只有一个产品 id - 但是，如果选择了多个，它们将是必要的。

【讨论】：