create  FUNCTION f_GetRentalMonths_double(@StartDate DATETIME ,@EndDate DATE)  
RETURNS DECIMAL(10,4)    
AS  
BEGIN  
  DECLARE @DiffMonths  DECIMAL(10,4) ,
           @DiffDays INT ,
          @BuZuMonths  DECIMAL (10,4)
  SET @DiffMonths=CAST(0 AS DECIMAL(10,4))
  while(@EndDate>=DATEADD(DAY,-1,DATEADD
 
(MONTH,1,@StartDate)))
  begin
  SET @DiffMonths=@DiffMonths+CAST(1 AS DECIMAL(10,4))
  SeT @StartDate=DATEADD(MONTH,1,@StartDate)
  end
  
--不足月天數
  SET @DiffDays = DATEDIFF(DAY,@StartDate,@EndDate) + 1
 --不足月天數/月天數，得到月份數
  SET @BuZuMonths=round( CAST(@DiffDays AS DECIMAL(10,2))/(CAST(365 AS DECIMAL(10,2
 
))/CASt(12 AS DECIMAL(10,2))),4)
  SET @DiffMonths=@DiffMonths+@BuZuMonths
  
  
  RETURN   @DiffMonths 
END  
GO