求助access 的Select语句

这是一个SQL Server里的一个语句，但是我想把转换成Access的SQL语句，但是Case语句，好像Access不识别，能不能用别的方法来代替这个Case，声明我不想用那个查询菜单下的SQL特定查询的那个，因为我用过了，好用但是速度太慢了，也不想用程序。只想用那个单纯的、最普通的[查询]。
下面是SQL语句，希望大家把他转换成Access同样功能的语句。谢谢:)
SELECT         
CASE WHEN Department=268 THEN 18
   WHEN Department=250 THEN 20
       ELSE Department END AS Department,
             Sum(CustomerSum) AS 合計,
             Sum(SaleSum) AS 売上額の合計
FROM            CustomerAmount INNER JOIN
                      Product ON
                      CustomerAmount.CompanyCD = Product.CompanyCD
                       AND
                      CustomerAmount.Department = Product.Department
                       AND
                      CustomerAmount.BusinessDate = Product.BusinessDate
   WHERE BusinessDate = DATEADD(d,-1,CAST(CONVERT(varchar,'2005/8/19',101) AS datetime))
GROUP BY Department
Order by Department

用iif代替case

SELECT         
iif(Department=268,18,iif(Department=250,20,Department)) AS Department,
             Sum(CustomerSum) AS 合計,
             Sum(SaleSum) AS 売上額の合計
FROM            CustomerAmount INNER JOIN
                      Product ON
                      CustomerAmount.CompanyCD = Product.CompanyCD
                       AND
                      CustomerAmount.Department = Product.Department
                       AND
                      CustomerAmount.BusinessDate = Product.BusinessDate
   WHERE BusinessDate = DATEADD(d,-1,CAST(CONVERT(varchar,'2005/8/19',101) AS datetime))
GROUP BY Department
Order by Department