T-SQL CTE问题（SQL Server 2008）

Question

为什么下面的查询在“NewAccounts”-CTE中没有名为Agentid的列时工作？

WITH 
NewAccounts AS (
    SELECT 3 as Newaccountid 
), 
MovedAUM AS (
    SELECT 1 AS Agentid, 2 as Col2 
) 
SELECT * FROM MovedAUM WHERE agentid IN (SELECT Agentid FROM NewAccounts) 

以下，修改后的查询将返回错误消息作为预期：

WITH 
NewAccounts AS (
    SELECT 3 AS newaccountid 
) 
SELECT Agentid FROM NewAccounts 

Answer 1

第一个工程，因为MovedAUM是在范围嵌套SELECT内。实际上，NewAccounts中的每一行都返回MovedAUM.AgentId。换句话说，WHERE子句无所作为 - 相当于WHERE 1 = 1。

此稍作修改的版本将失败：

WITH 
NewAccounts AS ( 
    SELECT 3 as Newaccountid 
), 
MovedAUM AS ( 
    SELECT 1 AS Agentid, 2 as Col2 
) 
SELECT * FROM MovedAUM WHERE agentid IN 
    (SELECT NewAccounts.Agentid FROM NewAccounts) 

Answer 2

因为你正在做明确的加盟，与

SELECT * FROM TAB1, TAB2 

Answer 3

SQL Server知道，它有AGENTID列，可以使用它，但它是不是你想要的，不要使用IN，使用EXISTS它会阻止这个

WITH 
NewAccounts AS (
    SELECT 3 as Newaccountid 
), 
MovedAUM AS (
    SELECT 1 AS Agentid, 2 as Col2 
) 
SELECT * FROM MovedAUM WHERE EXISTS (SELECT 1 
           FROM NewAccounts 
           where NewAccounts.Agentid = MovedAUM.agentid) 

另见：http://sqlservercode.blogspot.com/2007/04/you-should-never-use-in-in-sql-to-join.html