with t_unique( a ) as (
              select min(t1.a)
              from xt_test t1
              union all
              select next_a
              from t_unique t, lateral(select min(t1.a) next_a from xt_test t1 where t1.a>t.a) r
              where t.a is not null

with t_unique( job ) as (
              select min(t1.job)
              from emp t1
              union all
              select next_a
              from t_unique t, lateral(select min(t1.job) next_a from emp t1 where t1.job>t.job) r
              where t.job is not null
)
select nvl(job,'****') from t_unique

ANALYST
CLERK
MANAGER
PRESIDENT
SALESMAN
****