sql case列子 +ISNULL用法

SELECT     (CASE FDiscountType
                         WHEN 1 THEN ISNULL(fn_discount.FDiscountPercent, 1) * fn_income.FAmt - ISNULL(fn_discount.FDiscountAmt, 0)
                         WHEN 2 THEN fn_income.FAmt - ISNULL(fn_discount.FDiscountAmt, 0)
                  END) AS amt,
                      cs_courseGroups.FID, cs_courseGroups.FName, fn_income.FAmt
FROM         cs_courseGroups INNER JOIN
                      fn_income ON cs_courseGroups.FID = fn_income.FID LEFT OUTER JOIN
                      fn_discount ON cs_courseGroups.FID = fn_discount.FID
WHERE     ({ fn NOW() } >= fn_discount.FStartDatetime) AND ({ fn NOW() } <= fn_discount.FEndDatetime)

isnull ([字段]，默認值) 如果爲空返回的就是默認值