Update group of rows with sequence

Question

Hi guys,
I have no idea how to do an update of a table with a sequence value... but i don't want to increment the sequence for each row.
create sequence seq_num;
create table upd_group(
id number,
group_code varchar2(5),
seq_num number
insert into upd_group values(1,'a',null);
insert into upd_group values(2,'c',null);
insert into upd_group values(3,'a',null);
insert into upd_group values(4,'b',null);
insert into upd_group values(5,'a',null);
insert into upd_group values(6,'b',null);
commit;
select * from upd_group;
--doesn't work
update upd_group set seq_num = seq_num.nextval
where group_code in (select distinct group_code from upd_group)the expected result would be
all rows with group_code = 'a' would have N
all rows with group_code = 'b' would have N+1
all rows with group_code = 'c' would have N+2
and so on....
any idea how to do this?

Answer 1

Thanks for posting sample data!
If this is a one time shot, then analytics and MERGE might come to the rescue:
SQL> select * from upd_group order by group_code;
        ID GROUP    SEQ_NUM
         1 a
         3 a
         5 a
         4 b
         6 b
         2 c
6 rows selected.
SQL> --
SQL> --doesn't work
SQL> --update upd_group set seq_num = seq_num.nextval
SQL> --where group_code in (select distinct group_code from upd_group);
SQL> --
SQL> merge into upd_group a
  2  using ( select id
  3          ,      dense_rank() over (order by group_code) new_seq_num
  4          from   upd_group
  5         ) b
  6  on  (a.id = b.id)
  7  when matched then update set a.seq_num = b.new_seq_num;       
6 rows merged.
SQL> --
SQL> select * from upd_group order by group_code;
        ID GROUP    SEQ_NUM
         1 a              1
         3 a              1
         5 a              1
         4 b              2
         6 b              2
         2 c              3
6 rows selected.How a sequence could be able to take care of this in a query....that's interesting.
Depending on your database version you might opt for a virtual column....(11g stuff)