select chr(x), x  ascii FROM (select rownum x from all_tables)
 where x between 65 and 90

SQL> drop sequence s1
  2  /

Sequence dropped.

SQL> drop table tbl purge
  2  /

Table dropped.

SQL> create sequence s1
  2    start with 0
  3    minvalue 0
  4    maxvalue 675323
  5    cycle
  6  /

Sequence created.

SQL> create table tbl(
  2                   l   number,
  3                   val varchar2(5)
  4                  )
  5  /

Table created.

SQL> create or replace
  2    trigger tbl_bir
  3      before insert
  4      on tbl
  5      for each row
  6      declare
  7          v_seq number := s1.nextval;
  8      begin
  9          :new.val := chr(ascii('A') + trunc(trunc(v_seq / 999) / 26)) ||
 10                      chr(ascii('A') + mod(trunc(v_seq / 999),26)) ||
 11                      to_char(mod(v_seq,999) + 1,'FM000');
 12  end;
 13  /

Trigger created.

SQL> insert
  2    into tbl(l)
  3    select  level l
  4      from  dual
  5      connect by level <= 675400
  6  /

675400 rows created.

SQL> create index tbl_idx on tbl(l)
  2  /

Index created.

SQL> select  *
  2    from  tbl
  3    where l between &1 and &2
  4  /
Enter value for 1: 1
Enter value for 2: 5
old   3:   where l between &1 and &2
new   3:   where l between 1 and 5

         L VAL
---------- -----
         1 AA001
         2 AA002
         3 AA003
         4 AA004
         5 AA005

SQL> /
Enter value for 1: 995
Enter value for 2: 1005
old   3:   where l between &1 and &2
new   3:   where l between 995 and 1005

         L VAL
---------- -----
       995 AA995
       996 AA996
       997 AA997
       998 AA998
       999 AA999
      1000 AB001
      1001 AB002
      1002 AB003
      1003 AB004
      1004 AB005
      1005 AB006

11 rows selected.

SQL> /
Enter value for 1: 1995
Enter value for 2: 2005
old   3:   where l between &1 and &2
new   3:   where l between 1995 and 2005

         L VAL
---------- -----
      1995 AB996
      1996 AB997
      1997 AB998
      1998 AB999
      1999 AC001
      2000 AC002
      2001 AC003
      2002 AC004
      2003 AC005
      2004 AC006
      2005 AC007

11 rows selected.

SQL> /
Enter value for 1: 675320
Enter value for 2: 675329
old   3:   where l between &1 and &2
new   3:   where l between 675320 and 675329

         L VAL
---------- -----
    675320 ZZ995
    675321 ZZ996
    675322 ZZ997
    675323 ZZ998
    675324 ZZ999
    675325 AA001
    675326 AA002
    675327 AA003
    675328 AA004
    675329 AA005

10 rows selected.

SQL>