create table emp1 as select sal from emp;--only sal column suppose one table is having
         
update emp set comm=(select sal from emp1) ;

update emp set comm=sal;

update emp f set sal= to_number(rownum||'00');
or 
update emp set comm=sal;

DECLARE
II NUMBER:=0;
JJ NUMBER:=0;
 BEGIN
   FOR I IN ( select empno   FROM emp1) LOOP-- refering table
    JJ :=0;
    FOR J IN (select empno FROM emp) loop--updating table
       if II=JJ then  
       update emp  set comm= sal WHERE empno=i.empno;     
       EXIT;
       end if;
    JJ:=JJ+1;
    end loop;
   II:=II+1;
   END LOOP;
 END;

empno=j.empno

update emp set comm=rownum;--happening
update emp set comm=(select rownum from dual);--not happening

DECLARE
II NUMBER:=0;
JJ NUMBER:=0;
 BEGIN
   FOR I IN ( select empno FROM emp) LOOP --updating table 
    JJ :=0;
    FOR J IN (select sal FROM emp1) loop-- refering table
       if II=JJ then  
       update emp  set comm= j.sal WHERE empno=i.empno;     
       EXIT;
       end if;
    JJ:=JJ+1;
    end loop;
   II:=II+1;
   END LOOP;
 END;

update emp e set comm=(select sal from emp1 e1 where e1.empno=e.empno);

update emp set COMM='';

drop table test_emp;

create table test_emp as select sal from emp3;

select * from test_emp;

alter table test_emp add (rn number);

update test_emp  set rn=rownum;

create or replace function test_func(p_rn number) return number
is 
l_data number;
begin
select sal into l_data  from test_emp where rn=p_rn;
return l_data;
end;

update emp set comm=test_func(rownum) ;

commit;

select * from emp;