SQL更新表查詢不起作用

Question

我有一個SQL表需要使用另一個表中的數據更新。
這似乎是一個非常簡單的查詢，但我無法更新我的表。

CustID  FirstCommunicationDate SecondCommunicationDate 
20144  2013/02/01  2013/02/16 
20156  2013/02/10  2013/02/16 
20755  2013/02/09  2013/02/16 
20814  2013/04/14  2013/02/16 
20903  2013/06/12  2013/02/16 
21333  2013/06/21  2013/02/16 


CustID  CommunicationNum CommunicationDate 
20144  1   2013/02/16 
20144  1   2013/03/13 
20144  2   2013/04/18 
20903  1   2013/02/12 
20903  1   2013/03/19 
20903  2   2013/04/04 
21333  1   2013/02/16 
21333  1   2013/02/22 
21333  2   2013/04/29 

我的更新查詢：

UPDATE tblCommunication 
SET FirstCommunicationDate = (SELECT MAX(CommunicationDate) 
       FROM FieldData fd 
       WHERE CustID = fd.CustID 
       AND fd.CommunicationNum = 1) 
WHERE CustID IN (SELECT CustID FROM FieldData) 

我找的結果是：

CustID  FirstCommunicationDate SecondCommunicationDate 
20144  2013/03/13  2013/02/16 
20903  2013/03/19  2013/02/16 
21333  2013/02/22  2013/02/16 

我怎麼可以更新tblCommunication的日期？

Answer 1

update a 
set a.FirstCommunicationDate = b.MaxDate 
from tblCommunication as a 
inner join (select 
       CustID 
       ,MAX(CommunicationDate) as MaxDate 
      from FieldData 
      where CommunicationNum = 1 
      group by CustID) as b 
    on a.CustID = b.CustID 

Answer 2

試試這個

UPDATE tblCommunication 
SET FirstCommunicationDate = (SELECT MAX(CommunicationDate) 
       FROM FieldData fd 
       WHERE t.CustID = fd.CustID 
       AND fd.CommunicationNum = 1) 
FROM tblCommunication t 
WHERE t.CustID IN (SELECT CustID FROM FieldData)