Cross Apply and Outer Apply in SQL

DECLARE @Year INT

SET @Year = 2008

SELECT  ISNULL(ROUND(a.SalesAmount,0),0) AS CurrentSales

                     , ISNULL(ROUND(b.SalesAmount,0),0) As PreviousSales

                     ,a.MONTH As 'Month'

            FROM

                      (SELECT    SUM(Fs.SalesAmt) AS SalesAmount

                          ,DD.MonthName AS 'Month'

                           ,DD.MonthNumber

                              FROM    FactSales FS

                              JOIN     DimDate DD on DD.DateKey = FS.DateKey

                              WHERE    DD.Year = @Year

                              GROUP BY DD.MonthName

                           ,DD.MonthNumber

                          ) AS a

             OUTER APPLY

             --CROSS APPLY

                         (SELECT    Sum(Fs.SalesAmt) AS SalesAmount

                                      ,DD.MONTHNAME AS 'Month'

                                      ,DD.MonthNumber

                        FROM    FactSales FS

                        JOIN     DimDate DD ON DD.DateKey = FS.DateKey

                        WHERE     DD.Year = @Year - 1

                                    and   DD.MonthNumber = a.MonthNumber

                        GROUP BY DD.MONTHNAME

                           ,DD.MonthNumber

                         ) AS b

Consider ther is no data for year 2007, then on replacing ‘Outer Apply’ with ‘Cross Apply’ no data will be displayed. But using Outer Apply will display data for the year 2008 (current Sales) and 0 for 2007 (Previous Sales)