添加 WHERE 时 SQL CONVERT 不起作用

Question

使用： SQL Server 2008 Management Studio，T-SQL

问题描述：我基本上有两个带有AccountNumber 列的表。在一个表中存储为NUMERIC，在另一个表中存储为VARCHAR（不幸的是，这种数据类型差异无法更改）。

因此，为了保持一致性，为了将它们都转换为数值数据类型，我尝试的解决方案是仅在 VARCHAR 表中使用 LEFT JOIN，并将 AccountNumber 列转换为 NUMERIC。

效果很好，查询将运行。

然而， 我一添加WHERE 子句，它就会崩溃并出现如下所示的错误。如您所见，使用WHERE 子句，我基本上只是对特定的（NUMERIC）AccountNumber 进行查询搜索。

当来自LEFT JOIN 的AccountNumber 列是NUMERIC 时，为什么会出现此转换错误问题？

/*
Assumptions:
    A.AFIELD is numeric
    B.BFIELD is varchar

Result:
    Msg 8114, Level 16, State 5, Line 10
    Error converting data type varchar to numeric.
*/

SELECT A.AccountNumber, B.AccountNumber
FROM A
LEFT JOIN 
    (SELECT CONVERT(NUMERIC, B.AccountNumber) AccountNumber
     FROM B
     WHERE ISNUMERIC(B.AccountNumber) = 1) B ON B.AccountNumber = A.AccountNumber
WHERE A.AccountNumber = 1234567890
/*NOTE: I am searching the originally numeric AccountNumber field for a numeric, yet it's erroring out*/

Answer 1

你应该转换成VARCHAR:

select A.AccountNumber, B.AccountNumber
from A
left join
    (
    select AccountNumber
    from B
    where ISNUMERIC(B.AccountNumber)=1
    ) B on B.AccountNumber = CONVERT(VARCHAR(10),A.AccountNumber)
where A.AccountNumber = 1234567890;

Answer 2

也许我的评论“它告诉你 B 中有一个值不能转换为数字”不够清楚。试试这个：

select A.AccountNumber, B.AccountNumber
from A
left join
    (
    select CONVERT(numeric,B.AccountNumber) AccountNumber
    from B
    where ISNUMERIC(B.AccountNumber)=1
    ) B on coalesce(B.AccountNumber,0) = A.AccountNumber
where A.AccountNumber = 1234567890;

这是一个简单的演示：

DECLARE @t1 TABLE
  (
    accountnumber NUMERIC
  );
DECLARE @t2 TABLE
  (
    accountnumber VARCHAR(20)
  );

INSERT  @t1 ( [accountnumber] )
VALUES  ( 1 ),
        ( 2 ),
        ( 3 );

INSERT  @t2 ( [accountnumber] )
VALUES  ( '1' ),
        ( '3' ),
        ( 'NonConvertible' );


SELECT  A.accountnumber, B.accountNumber
FROM    @t1 AS [A]
LEFT JOIN (
            SELECT
                CASE 
                    -- Still not 100% safe because ISNUMERIC isn't
                    WHEN ISNUMERIC(B.accountnumber) = 1
                    THEN CONVERT(NUMERIC, accountnumber) 
                    ELSE CONVERT(NUMERIC, NULL)
                END AS accountNumber
            FROM    @t2 AS [B]
            WHERE   ISNUMERIC(accountNumber) = 1
--          ) B ON B.accountNumber = A.accountnumber
          ) B ON COALESCE(B.accountNumber, 0) = A.accountnumber
WHERE   A.accountnumber = 1;