create table tab1 ( a  number, b  varchar2(10));
create index idx_a on tab1(a);
insert into tab1 (a, b) values (750, 'aaa');
insert into tab1 (a, b) values (750, 'aaa');
insert into tab1 (a, b) values (26, 'bbb');
insert into tab1 (a, b) values (3450, 'ccc');
insert into tab1 (a, b) values (3450, 'ccc');
insert into tab1 (a, b) values (12, 'q');
insert into tab1 (a, b) values (12, 'q');
commit;

create table tab2( c  number, d  varchar2(10));

insert into tab2(c) values (7503546);
insert into tab2(c) values (75043111);
insert into tab2(c) values (7507895);
insert into tab2(c) values (7501478);
insert into tab2(c) values (264545);
insert into tab2(c) values (26777);
insert into tab2(c) values (26147);
insert into tab2(c) values (34506273);
insert into tab2(c) values (3450543);
insert into tab2(c) values (128887);
insert into tab2(c) values (1294387);
commit;

declare
v_c  number;
v_d  varchar2(10);
v_r rowid;
i number;
cursor v_cursor is 
select rowid,c from tab2;

 begin
  open v_cursor;
 loop
  begin
   fetch v_cursor into v_r,v_c;
   i:=length(v_c);
   v_d:=null;
   
   while i>1 and v_d is null loop 
             --dbms_output.put_line(v_c);
        begin     
        select nvl(b,0) into v_d from tab1 where a=v_c and rownum<2;
          exception when others then null;   
        end;
        i:=i-1;      
        v_c:= substr(v_c,1,i);
   end loop;
   if v_d is not null then
     begin
      update tab2 set d=v_d where rowid=v_r;
     end;
   end if;
  end; 
     exit when v_cursor%NOTFOUND;
   end loop;
  close v_cursor;
 end;    

describe tab2;