how to Descending Sql query

 Dear friends i have four columns date column in a table.
Ex: ticketprintdate,testprintdate,checkdate,errordate. if one column have date another 3 column must be null.what i need based on 4 column i need to  sorting   the date.
(dd/mm/yyyy) 
ticket date     test print date   check date  error date
--------------------------------------------------------------------------------
01/ 01/2008         null                   null                 null
null                    08/01/2008           null               null
null      &n ...