Oracle 11g Release 2

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with payers(p_number, name) as (
    select 44545558, 'sth1' from dual union all
    select 44545559, 'sth2' from dual union all
    select 44545560, 'sth3' from dual union all
    select 44545561, 'sth4' from dual union all
    select 44545562, 'sth5' from dual ),
  trans(t_num, p_number, timestamp) as (
    select 1, 44545558, timestamp '2018-05-30 19:22:05' from dual union all
    select 2, 44545558, timestamp '2017-05-30 19:22:05' from dual union all
    select 3, 44545558, timestamp '2016-05-30 19:22:05' from dual union all
    select 4, 44545559, timestamp '2018-05-30 19:22:05' from dual union all
    select 6, 44545559, timestamp '2016-05-30 19:22:05' from dual )
select p.p_number, p.name, t.t_num, t.timestamp
  from payers p
  left join (select p_number, t_num, timestamp,
                    row_number() over (partition by p_number order by timestamp desc) rn 
               from trans t 
               where t.timestamp <= date '2017-07-31') t
         on p.p_number = t.p_number and rn = 1
  where p.p_number between 44545558 and 44545562

P_NUMBER	NAME	T_NUM	TIMESTAMP
44545558	sth1	2	30-MAY-17 07.22.05.000000000 PM
44545559	sth2	6	30-MAY-16 07.22.05.000000000 PM
44545560	sth3	null	null
44545561	sth4	null	null
44545562	sth5	null	null