Merge Multiple Rows into a Single Row in SQL Server

      In Some times Need Multiple Rows into single row and combine column SQL.

   this is My Table Studenttbl I Shown All RollNo And I Merge to Single Row. for Use Following SQL Query.  

declare @Roll as varchar(max)=''

select @Roll= substring(

        (select ' ,'+RollNo AS 'data()' from tbl_Studentdtl for xml path(''))

       ,3, 255)

          select @Roll RoleNo