create table Main(Main_id number, id_num number, desc varchar2(50), stage_code number);
insert into department values(1,11,'X',4);
insert into department values(2,22,'Y',10);
insert into department values(3,33,'Z',15);
create table Task(task_id number, status varchar2(50));
insert into Task values(1,'A');
insert into Task values(2,'B');
insert into Task values(3,'C');
select m.id_num, m.desc,
case
when m.stage_code between 0 to 10 then t.status
else null
end as stage_code
from Main m
join Task t on t.task_id = m.main_id;