使用来自多个表的值填充单个列

Question

我有三个表如下：

订单表

| OrderID  | Type      | ItemID |
+----------+-----------+---------+
| 0001     | Car       | A001    |
| 0002     | Motorbike | A001    |

Car_Table

| ItemID  | ItemDesc   |
+---------+------------+
| A001    | Honda Jazz |

Motorbike_Table

| ItemID  | ItemDesc   |
+---------+------------+
| A001    | Yamaha F1Z |

我需要通过添加一个名为ItemDesc的列将最后两个表合并到第一个表中。

有什么方法可以根据Type列的值进行查询以从Car_Table或Motorbike_Table填充ItemDesc列？ 回答：

SELECT Order_Table.OrderID, Order_Table.Type, Order_Table.ItemID,
IIF(Order_Tbl.Type = "Car", Car_Tbl.ItemDesc, Motorbike_Tbl.ItemDesc) AS ItemDesc
FROM Order_Table 
LEFT JOIN Car_Table ON Order_Table.ItemID = Car_Table.ItemID
LEFT JOIN Motorbike_Table ON Order_Table.ItemID = Motorbike_Table.ItemID;

谢谢！

Answer 1

您可以使用条件聚合来获得所需的结果，当Type = Car时从Car_table中选择一个值，当 Type Type = Motorbike Motorbike_table时从 Motorbike_table 中选择一个值：

SELECT Order_Table.OrderID, Order_Table.Type, Order_Table.ItemID,
IIF(Order_Tbl.Type = "Car", Car_Tbl.ItemDesc, Motorbike_Tbl.ItemDesc) AS ItemDesc
FROM Order_Table 
LEFT JOIN Car_Table ON Order_Table.ItemID = Car_Table.ItemID
LEFT JOIN Motorbike_Table ON Order_Table.ItemID = Motorbike_Table.ItemID;

Output：

OrderID     Type        ItemID  ItemDesc
1           Car         A001    Honda Jazz
2           Motorbike   A001    Yamaha F1Z

dbfiddle 上的演示

Answer 2

您可以使用UNION ALL ：

SELECT o.OrderID, o.Type, o.ItemID, c.ItemDesc
FROM Order_Table o INNER JOIN
     Car_Table c
     ON c.ItemID = o.ItemID
WHERE o.Type = 'Car'
UNION ALL
SELECT o.OrderID, o.Type, o.ItemID, m.ItemDesc
FROM Order_Table o INNER JOIN
     Motorbike_Table m
     ON c.ItemID = m.ItemID
WHERE o.Type = 'Motorbike';

MS Access 对连接很挑剔，所以我认为以下方法行不通：

SELECT o.OrderID, o.Type, o.ItemID,
       NZ(c.ItemDesc, m.ItemDesc) AS ItemDesc
FROM (Order_Table o LEFT JOIN
      Car_Table c
      ON c.ItemID = o.ItemID AND o.Type = 'Car'
     ) LEFT JOIN
     Motorbike_Table m
     ON m.ItemID = o.ItemID AND o.type = 'Motorbike'