SELECT a.[Acc No], a.[key], a.PayDate, a.[Code]
    ,TaxAmount=CASE WHEN Code1 IS NULL THEN -c.TaxAmount 
                    WHEN Code1 = Code2 THEN b.Order2
                    ELSE 0 END 
    ,[TaxRate]=CASE WHEN Code1 IS NULL THEN c.[TaxRate] 
                    WHEN Code1 = Code2 THEN c.[TaxRate]
                    ELSE 0 END 
    ,[GrossAmount]=CASE WHEN Code1 IS NULL THEN -c.[GrossAmount] 
                    WHEN Code1 = Code2 THEN b.Order1
                    ELSE b.Order1 END 
FROM dbo.datasheet a
CROSS APPLY (
    VALUES (NULL, NULL, NULL, NULL) 
        ,(NewOrder_Code, [NewOrder ( code 6)], [TaxOnNewOrder_Code], [TaxOnNewOrder ( code 06)]) 
        ,([NewOrderDiv_Code ], [NewOrderDiv_Code ( code 37)], NULL, NULL)) b (Code1, [Order1], Code2, [Order2])
INNER JOIN dbo.originalsheet c
    ON a.[Acc No] = c.[AccountNo] AND a.[key] = c.[key] AND a.PayDate = c.PayDate AND
        a.[Code] = c.[Code]