sql-server - レコードがまだ存在しない場合はテーブルに挿入

Question

何らかの理由で、これにより「重複レコードをテーブルに挿入できません」というエラーが表示されます。

INSERT  INTO [DMS].[dbo].[Deductions]
        (
         CustomerID,
         DeductionCode,
         DeductionDescription
        )
        SELECT  b.CustomerID,
                b.AdjustmentReason,
                b.AdjustmentReason
        FROM    @CreditDebitAdjustmentDetail b
        WHERE   NOT EXISTS ( SELECT 1
                             FROM   [DMS].[dbo].[Deductions]
                             WHERE  CustomerID = b.CustomerID
                                    AND DeductionCode = b.AdjustmentReason )

奇妙なことに、私はそれを次のようにテストしました：

DECLARE @CreditDebitAdjustmentDetail TABLE
        (
         CustomerID INT,
         AdjustmentReason VARCHAR(50)
        )

INSERT  INTO @CreditDebitAdjustmentDetail
        (CustomerID, AdjustmentReason)
VALUES  (143, -- CustomerID - int
         '024'  -- AdjustmentReason - varchar(50)
         )

INSERT  INTO [DMS].[dbo].[Deductions]
        (
         CustomerID,
         DeductionCode,
         DeductionDescription
        )
        SELECT  b.CustomerID,
                b.AdjustmentReason,
                b.AdjustmentReason
        FROM    @CreditDebitAdjustmentDetail b
        WHERE   NOT EXISTS ( SELECT 1
                             FROM   [DMS].[dbo].[Deductions]
                             WHERE  CustomerID = b.CustomerID
                                    AND DeductionCode = b.AdjustmentReason )

レコードが既に存在するため、テーブルに挿入しません。

ここで何か不足していますか？

編集-これを行うことで修正したと思いましたが、それでも同じエラーが発生します:

INSERT  INTO [DMS].[dbo].[Deductions]
        (
         CustomerID,
         DeductionCode,
         DeductionDescription
        )
        SELECT  a.CustomerID,
                a.AdjustmentReason,
                a.AdjustmentReason
        FROM    @CreditDebitAdjustmentDetail a
        WHERE   NOT EXISTS ( SELECT 1
                             FROM   [DMS].[dbo].[Deductions] b
                             WHERE  a.CustomerID = b.CustomerID
                                    AND a.AdjustmentReason = b.DeductionCode )