how to Format wise update in oracle database

select * from project_requirement where req_date='02/01/19'
update project_requirement set req_date=to_date(to_char(req_date,'DD/MM/RR'),'DD-MON-RR') where req_date='02/01/19'


select req_date from project_requirement_copy1 where req_date='02/01/19';
select to_date(req_date,'dd/mm/rr') from project_requirement_copy1 where req_date='02/01/19';
select req_date from project_requirement_copy1 where req_date='02-JAN-19';
select to_char(to_date(req_date,'dd/mm/rr'),'dd-mon-rr') from project_requirement_copy1 where  req_date='02/01/19';

update project_requirement_copy1
set  req_date=to_char(to_date(req_date,'dd/mm/rr'),'dd-mon-rr') where req_date='02/01/19';

select * from project_requirement_copy1  where req_date='25/1/19';

select SUBSTR(req_date, 1, 2)||''||SUBSTR(req_date, 3, 3)||''||SUBSTR(req_date, 6, 3) from project_requirement_copy1 where req_date='02/01/19';

--02/01/19

select req_date from PROJECT_REQUIREMENT_COPY1;
select to_date(req_date,'DD/MM/RR') from PROJECT_REQUIREMENT_COPY1 where req_date=SUBSTR(req_date, 1, 2)||''||SUBSTR(req_date, 3, 3)||''||SUBSTR(req_date, 6, 3);

select req_date from PROJECT_REQUIREMENT;
update PROJECT_REQUIREMENT set req_date=to_date(req_date,'DD/MM/RR') 
where req_date=SUBSTR(req_date, 1, 2)||''||SUBSTR(req_date, 3, 3)||''||SUBSTR(req_date, 6, 3);

select count(req_date) from PROJECT_REQUIREMENT_COPY1 where req_date=SUBSTR(req_date, 1, 2)||''||SUBSTR(req_date, 3, 3)||''||SUBSTR(req_date, 6, 3);

select req_date from PROJECT_REQUIREMENT_COPY1 where req_date=SUBSTR(req_date, 1, 2)||''||SUBSTR(req_date, 3, 4)||''||SUBSTR(req_date, 6, 3);

select SUBSTR(req_date, 1, 2)||''||SUBSTR(req_date, 3, 4)||''||SUBSTR(req_date,7, 3)  from PROJECT_REQUIREMENT_COPY1 where req_date='01-MAR-21';

select req_date from PROJECT_REQUIREMENT_COPY1 where req_date= to_date(req_date,'DD-MON-RR');---SUBSTR(req_date, 1, 2)||''||SUBSTR(req_date, 3, 4)||''||SUBSTR(req_date,7, 3);
01-OCT-19



update PROJECT_REQUIREMENT a
set a.req_date1= (select b.req_date from PROJECT_REQUIREMENT b where a.project_no=b.project_no and a.req_no=b.req_no and project_no is not null)