CREATE PROC sp_Factorial
    @InPara int,
    @OutPara int OUTPUT
    AS
    DECLARE @InCalc int
    DECLARE @OutCalc int
    IF @InPara != 1
    BEGIN
          SELECT @InCalc = @InPara - 1

          EXEC sp_Factorial @InCalc, @OutCalc OUTPUT

          SELECT @OutPara = @InPara * @OutCalc
    END
    ELSE
    BEGIN
          SELECT  @OutPara = 1
    END
    RETURN
    GO

    DECLARE @CalcOut int
    DECLARE @CalcIn int
    SELECT @CalcIn = 6
    EXEC sp_Factorial @CalcIn, @CalcOut OUTPUT

    PRINT  ' factorial  of ' + CAST(@CalcIn AS varchar)  + 'is ' + CAST(@CalcOut AS varchar)
    GO