如何使用子查詢中的多個表達式創建一個SQL存儲過程

Question

我有一個Orders表，我想從Orders表中取出一些列，並從兩個OrderDetails表中的一個取決於Orders表中列的值。

訂單

OrderID Pk int 
OrderDetails_A_ID 
OrderDate 
OrderedById 

OrderDetails_A

OrderDetails_A_ID Pk int 
OrderID Fk int 
Qty_A 
ItemDesc_A 

OrderDetails_B

OrderDetails_B_ID Pk int 
OrderID Fk int 
Qty_B 
ItemDesc_B 

這裏是我硝酸鉀w不起作用，但我想要做的。

SELECT O.OrderDate,O.OrderedByID, 
CASE WHEN O.OrderDetails_A_ID IS NULL 
    THEN 
    SELECT B.ItemDesc_B as ItemDesc,B.Qty_B as Qty 
    ELSE 
    SELECT A.ItemDesc_A as ItemDesc,A.Qty_A as Qty 
    END 
FROM Orders as O 

Answer 1

您沒有在您的示例中包含任何JOIN條件，因此我也將它們從中省略。

SELECT O.OrderDate,O.OrderedByID, 
CASE WHEN O.OrderDetails_A_ID IS NULL THEN B.ItemDesc_B ELSE A.ItemDesc_A END as ItemDesc, 
CASE WHEN O.OrderDetails_A_ID IS NULL THEN B.Qty_B ELSE A.Qty_A END as Qty 
FROM Orders as O 

Answer 2

您可以合併或使用聯合使用內部聯接。

使用合併，您可以控制每個字段。使用聯合，您不必一遍又一遍地指定確切的列。

合併：

select o.OrderID, COALESCE(a.Qty_A, a.Qty_B) as Qty 
from Orders o 
left outer join Order_Detail_A a on o.OrderID = a.OrderID and o.OrderDetails_A_ID is not null 
left outer join Order_Detail_B b on o.OrderID = b.OrderID and o.OrderDetails_A_ID is null 

工會：

select o.OrderID, a.Qty_A as Qty 
from Orders o 
inner join Order_Detail_A a on o.OrderID = a.OrderID and o.OrderDetails_A_ID is not null 
union all 
select o.OrderID, b.Qty_B as Qty 
from Orders o 
inner join Order_Detail_B b on o.OrderID = b.OrderID and o.OrderDetails_A_ID is null 

Answer 3

select 
    o.OrderDate, 
    o.OrderedByID, 
    case when o.OrderDetails_A_ID is null then b.ItemDesc_B else a.ItemDesc_A end as ItemDesc, 
    case when o.OrderDetails_A_ID is null then b.Qty_B else a.Qty_A end as Qty 
FROM 
    Orders as O 
    left join OrderDetails_A a on a.OrderID = o.OrderID and o.OrderDetails_A_ID is not null 
    left join OrderDetails_B b on b.OrderID = o.OrderID and o.OrderDetails_A_ID is null