Specify a cross join. Because a cross join is functionally the same as a Cartesian product join, the cross join syntax can be substituted for the conventional join syntax.

proc sql;
   title 'All Possible Connections';
   select f1.Dest, case
                      when f1.dest ne ' ' then 'to and from'
                   end,
       f2.Dest
       from flights as f1 cross join flights as f2
       where f1.dest < f2.dest
       order by f1.dest;