QL> select * from t1;

        ID       K_ID
---------- ----------
         1        100
         2        200
         3        300

SQL> select * from t2;

        ID  PARENT_ID
---------- ----------
       100         11
       200         22

SQL> update (select t1.k_id,t2.parent_id from t1,t2 where  t1.k_id = t2.id) set k_id=parent_id;

2 rows updated.

SQL> select * from t1;

        ID       K_ID
---------- ----------
         1         11
         2         22
         3        300

SQL> update t1 set k_id=(select parent_id from t2 where t1.k_id = t2.id)
  2  where exists (select 1 from t2 where t1.k_id = t2.id)
  3  /

2 rows updated.

SQL> select * from t1;

        ID       K_ID
---------- ----------
         1         11
         2         22
         3        300

SQL> ed
Wrote file afiedt.buf

  1  declare
  2   cursor c1 is select t1.k_id,t2.parent_id from t1,t2 where  t1.k_id = t2.id for update of t1.k_id;
  3  begin
  4   for r in c1 loop
  5     update t1 set k_id=r.parent_id where current of c1;
  6   end loop;
  7   --rollback;
  8   --commit;
  9* end;
SQL> /

PL/SQL procedure successfully completed.

SQL> select * from t1;

        ID       K_ID
---------- ----------
         1         11
         2         22
         3        300

SQL> ed
Wrote file afiedt.buf

  1  begin
  2   for r in (select * from t2) loop
  3     update t1 set k_id=r.parent_id where t1.k_id = r.id;
  4   end loop;
  5   --rollback;
  6   --commit;
  7* end;
SQL> /

PL/SQL procedure successfully completed.

SQL> select * from t1;

        ID       K_ID
---------- ----------
         1         11
         2         22
         3        300