select *
  from (select <columns>,
               count() over (partition by "это поле") cnt
          from my_table)
 where cnt = 1

select c.*,
    (select count(*) from city c1 where c1.descr = c.descr) as descr_count,
    (select count(*) from city c1 where c1.name = c.name) as name_count
from city c
having descr_count = 1 /* или name_count = 1 */