Oracle 11g Release 2

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create table emp_request(emp_id, due_date) as (

select 9, date '2019-01-12' from dual union all

select 7, date '2019-01-19' from dual union all

select 2, date '2019-02-19' from dual union all

select 3, date '2019-02-19' from dual union all

select 7, date '2019-02-24' from dual union all

select 5, date '2019-03-07' from dual union all

select 7, date '2019-03-30' from dual);

7 rows affected

select to_char(to_date(mth_num, 'MM'), 'MONTH') month, nvl(cnt, 0) cnt

from (

select count(emp_id) as cnt, to_char(due_date, 'mm') mth_num

from emp_request where due_date is not null

group by to_char(due_date, 'mm')) e

right join (

select to_char(level, 'fm00') mth_num

from dual connect by level <= 12) m using (mth_num)

order by mth_num