外键关系总和并处理空值

Question

我有一个查询，它计算我的收据中的项目并返回它们的数据以生成报告。我有一些项目被退回的情况，因此在获取此数据时，我必须通过使用 return_item_id 获取所有操作来考虑这一点。

物品表：
身份证
名字

操作表：
return_item_id
金额

ReceiptItem 表：
item_id
价格
金额

select
cps_item.name as "item.name",
ROW_NUMBER () OVER (ORDER BY e.create_ts) as "row",
e.amount - SUM(cps_operation.amount) as "amount",
e.price as "price",
e.amount * e.price as "sum"
from cps_receipt_items e  
left join cps_item ON e.item_id = cps_item.code
left join cps_operation ON e.id = cps_operation.return_item_id
where e.receipt_id = ${sell}
and e.item_id is not null
group by cps_item.name, e.amount, e.price, e.create_ts;

我想要的预期结果是正确显示所有金额，但是当 SUM(cps_operation.amount) 返回 Null 时会发生什么，因为没有返回操作它返回空值，而不是只显示没有 SUM 结果的金额（ cps_operation.amount)。

Answer 1

使用合并

select
cps_item.name as "item.name",
ROW_NUMBER () OVER (ORDER BY e.create_ts) as "row",
e.amount - coalesce(SUM(cps_operation.amount),0) as "amount",
e.price as "price",
e.amount * e.price as "sum"
from cps_receipt_items e  
left join cps_item ON e.item_id = cps_item.code
left join cps_operation ON e.id = cps_operation.return_item_id
where e.receipt_id = ${sell}
and e.item_id is not null
group by cps_item.name, e.amount, e.price, e.create_ts;