linq union后怎么按时间排序

var query1 =
                (from pl in sds.Tables[0].AsEnumerable()
                 where "12" == pl.Field<string>("SysType")
                 select pl).Union
                  (from pl in sds.Tables[0].AsEnumerable()
                   where "22" == pl.Field<string>("SysType")
                   select pl).Union
                   (from pl in sds.Tables[0].AsEnumerable()
                    where "32" == pl.Field<string>("SysType")
                    select pl);