总和值为空时SQL返回行

我有两张桌子。一个带物料清单，一个带采购订单。现在，我想显示产品的完整物料清单以及采购表中的订单总金额

**Billofmaterials**
BOM_ID   BOM_Prod_id     BOM_item_id
1        5               11
2        5               13
3        6               11
4        6               15
5        6               20

示例产品id（产品id）6有3项（id 11、15和20）

现在我得到了以下SQL

SELECT 
BOM_item_id,
SUM(DISTINCT purchasing.PU_amount) as total_on_order
FROM Billofmaterials
LEFT JOIN purchasing
ON Billofmaterials.BOM_item_id= purchasing.PU_item_id
AND purchasing.PU_status != 'Received'
AND BOM_prod_id = 6
GROUP BY BOM_item_id

此查询返回以下内容：

**Query result**
BOM_item_id   total_on_order
11            750             
20            600                       

因为BOM\u item\u id 15只有一个收到的采购订单，所以它不返回值。现在我还想重新运行BOM\u item\u id 15，但总订单为0，如：

**Expected result**
BOM_item_id   total_on_order
11            750        
15            0      
20            600                       

---

我需要使用什么SQL特性/函数来获得预期的结果？

您可以尝试以下方法-

SELECT BOM_item_id,coalesce(total_on_order,0) as total_on_order
FROM Billofmaterials left join
(
select PU_item_id,SUM(purchasing.PU_amount) as total_on_order
from purchasing
where purchasing.PU_status != 'Received'
group by PU_item_id
) purchasing
ON Billofmaterials.BOM_item_id= purchasing.PU_item_id
where BOM_prod_id = 6

是的，非常感谢！我在末尾添加了“按BOM\u item\u id分组”以对项目进行分组。

SELECT BOM_item_id,coalesce(total_on_order,0) as total_on_order
FROM Billofmaterials left join
(
select PU_item_id,SUM(purchasing.PU_amount) as total_on_order
from purchasing
where purchasing.PU_status != 'Received'
group by PU_item_id
) purchasing
ON Billofmaterials.BOM_item_id= purchasing.PU_item_id
where BOM_prod_id = 6