CREATE FUNCTION CalcDaysYearBasis(@iDaysCode INT) 
RETURNS INT
AS BEGIN

    RETURN CAST (
        CASE @iDaysCode
            WHEN  1  THEN  365 
            WHEN  2  THEN  360 
            WHEN  3  THEN  360 
            ELSE NULL 
        END 
    AS INT)

END

CREATE FUNCTION CalcDaysNumber(
    @dtFrom DATETIME, @dtTo DATETIME, @iDaysCode INT) 
RETURNS INT
AS BEGIN

    RETURN CAST(
        CASE 
            WHEN (@dtTo Is NULL) OR (@dtFrom Is NULL) THEN NULL
            WHEN @iDaysCode= 1  THEN CAST(@dtTo as INT) - CAST(@dtFrom as INT)
            WHEN @iDaysCode= 2  THEN CAST(@dtTo as INT) - CAST(@dtFrom as INT)
            WHEN @iDaysCode= 3  THEN CASE 
                WHEN  30  - DAY(@dtFrom)> 0  
                THEN  30  - DAY(@dtFrom)
                ELSE  0  
            END 
            - CASE 
                WHEN  30  - DAY(@dtTo) >  0 
                THEN  30  - DAY(@dtTo) 
                ELSE  0 
            END
            +  30  * (MONTH(@dtTo)-MONTH(@dtFrom))
            +  360  * (YEAR(@dtTo)-YEAR(@dtFrom))
        END 
    AS INT)

END

CREATE FUNCTION CalcInterest(
    @dbAmount MONEY, @dbInterestRate DECIMAL( 12 , 8 ), 
    @dtFrom DATETIME, @dtTo datetime, @iDaysCode int) 
RETURNS MONEY
AS BEGIN

RETURN 
    CAST ( 
        ROUND (
            (@dbAmount*@dbInterestRate/ 100 )
            * dbo.CalcDaysNumber(@dtFrom, @dtTo, @iDaysCode) 
            / dbo.CalcDaysYearBasis(@iDaysCode), 
             2 
        ) AS MONEY
    )

END