あなたの例は、カスタム表現を使用して日付を文字列として保存することを示しています-日付形式に準拠していません。したがって、日付を日付データ型に変換して、それに応じて順序付けできるようにする必要があります。
SQL> create table TB_DATES
2 (
3 DATE_VALUE VARCHAR2(11)
4 )
5 /
Table created
SQL>
SQL> insert into tb_dates(date_value)
2 select '5th Feb 11' from dual union all
3 select '2nd Jan 11' from dual union all
4 select '6th Feb 11' from dual union all
5 select '11th Jan 11' from dual
6 ;
4 rows inserted
SQL> commit;
Commit complete
SQL> select * from tb_dates;
DATE_VALUE
-----------
5th Feb 11
2nd Jan 11
6th Feb 11
11th Jan 11
SQL> alter table tb_dates add srno number;
Table altered
QL> select * from tb_dates;
DATE_VALUE SRNO
----------- ----------
5th Feb 11 null
2nd Jan 11 null
6th Feb 11 null
11th Jan 11 null
SQL> merge into tb_dates t
2 using(
3 select rownum rn
4 , Date_value
5 , rid
6 from ( select to_date(concat(lpad(dy, 2, '0'), tr), 'dd Mon yy') dt
7 , Date_value
8 , rid
9 from (select regexp_substr(substr(Date_value, 1, regexp_instr(date_value, '[[:space:]]') - 1), '[[:digit:]]+') dy
10 , substr(Date_value, regexp_instr(date_value, '[[:space:]]'), length(Date_value)) tr
11 , Date_value
12 , rowid rid
13 from tb_dates)
14 order by 1
15 ) ) q
16 on (q.rid = t.rowid)
17 when matched
18 then update set t.srno = q.rn
19 ;
Done
SQL> commit;
Commit complete
select * from tb_dates;
DATE_VALUE SRNO
----------- ----------
5th Feb 11 3
2nd Jan 11 1
6th Feb 11 4
11th Jan 11 2
SQL> select * from tb_dates order by srno;
DATE_VALUE SRNO
----------- ----------
2nd Jan 11 1
11th Jan 11 2
5th Feb 11 3
6th Feb 11 4