不能內部加入到嵌入式查詢中

Question

我正在使用Northwind精簡版數據庫，並試圖提取哪些銷售人員獲得了多少。

所以，我的查詢做工精細的內部零件，在我得到員工ID的總費用清單：

select orderCosts."Employee ID", sum(orderCosts.Cost) as OrderCost from 
    (
     select Orders."Order Id",orders."Employee ID", orders."Customer ID", "Order Details"."Unit Price" * "Order Details"."Quantity" as Cost from orders 
     inner join "Order Details" 
     On Orders."Order ID" = "Order Details"."Order ID" 
    ) orderCosts 
    group by ordercosts."Employee ID" 

我的想法是好的，我會再嘗試加入到僱員表，這似乎並不複雜，但現在我得到一個"The table aliases must be unique. [Name of duplicate alias = employeePerformance]"錯誤。我嵌入式和命名的表，貌似我面前，像這樣：

select Employees."FirstName", Employees."LastName", employeePerformance.OrderCost 
from 
(
    select orderCosts."Employee ID", sum(orderCosts.Cost) as OrderCost from 
    (
     select Orders."Order Id",orders."Employee ID", orders."Customer ID", "Order Details"."Unit Price" * "Order Details"."Quantity" as Cost from orders 
     inner join "Order Details" 
     On Orders."Order ID" = "Order Details"."Order ID" 
    ) orderCosts 
    group by ordercosts."Employee ID" 
) employeePerformance 
inner join employeePerformance 
On Emplyees."Employee ID" = employeePerformance."Employee ID" 

我可以看到，我只聲明empployeePerformance一次，然後試圖內連接到它，那麼它爲什麼認爲我有兩次？

我對SQL很陌生，所以如果我正在做一些傻事，請客氣一點！

Answer 1

我想你應該對員工添加FROM，在您的查詢的子查詢的聯接（如果我深知）

select Employees."FirstName", Employees."LastName", employeePerformance.OrderCost 
from Employees 
INNER JOIN 
(
    select orderCosts."Employee ID", sum(orderCosts.Cost) as OrderCost from 
    (
     select Orders."Order Id",orders."Employee ID", orders."Customer ID", "Order Details"."Unit Price" * "Order Details"."Quantity" as Cost from orders 
     inner join "Order Details" 
     On Orders."Order ID" = "Order Details"."Order ID" 
    ) orderCosts 
    group by ordercosts."Employee ID" 
) employeePerformance 

On Employees."Employee ID" = employeePerformance."Employee ID" 

原因，您從上employeesPerformance和employeesperformance內加入=>重複名稱，這是不允許的。

順便說一句，你的查詢可以廣泛簡化，我想：

SELECT e."FirstName", e."LastName", sum(od."Unit Price" * od."Quantity") as OrderCosts 
FROM employees e 
inner join orders o on o."Employee ID" = e."Employee ID" 
inner join "Order Details" od on od."Order ID" = o."Order ID" 
group by e."Employee ID", e."FirstName", e."LastName" 

使用左連接，而不是內部聯接（和凝聚在單價和數量），如果你希望員工沒有訂單/訂單的詳細信息。

SELECT e."FirstName", e."LastName", sum(coalesce(od."Unit Price", 0) * coalesce(od."Quantity", 0)) as OrderCosts 
FROM employees e 
left join orders o on o."Employee ID" = e."Employee ID" 
left join "Order Details" od on od."Order ID" = o."Order ID" 
group by e."Employee ID", e."FirstName", e."LastName"