Update statement using correlated partition over query - help asked please.

Question

A simple table:
SQL> desc vs
Name Null? Type
COL1 VARCHAR2(10)
COL2 NUMBER
SQL> select * from vs;
COL1 COL2
A
A
B
B
B
C
D
D
E
E
E
E
E
13 rows selected.
I want to Update col2 with sequence number grouped by col1, in one step.
EG. The seqno returned by:
select col1, row_number() over (partition by col1 order by rowid) seqno
from vs;
COL1 SEQNO
A 1
A 2
B 1
B 2
B 3
C 1
D 1
D 2
E 1
E 2
E 3
E 4
E 5
13 rows selected.
And UPDATE col2 with the seqno.
The following worked:
merge into vs v1
using (select row_number() over (partition by col1 order by rowid)seqno,rowid from vs) v
on (v1.rowid=v.rowid)
when matched then update set v1.col2 = v.seqno
when not matched then insert (v1.col1,v1.col2)
values('Z',null);
13 rows merged.
SQL> select * from vs;
COL1 COL2
A 1
A 2
B 1
B 2
B 3
C 1
D 1
D 2
E 1
E 2
E 3
E 4
E 5
13 rows selected.
Can someone help with a simpler UPDATE or WITH ... statement?
==============
This isn't urgent, just frustrated with syntax ;-)
Best wishes of the season.

Answer 1

Thank you for the reply.
I've figured it out (with help).
update vs a
set col2 =
(select seq from
select rowid, row_number() over (partition by col1 order by rowid) as seq
from vs
) b
where a.rowid = b.rowid
To complicate things (for my own interest)
If some of the col2 were not null, and I had to sequence the null rows starting above the maximum value already used, I got this to work.
SQL> select * from vs;
COL1 COL2
A 1
A 2
B 1
B 2
B 3
C
D 1
D 2
E 1
E 10
E
E
E
update vs a
set col2 =
(select seq from
select rowid, row_number() over (partition by col1 order by rowid)+
(select max(nvl(col2,0)) from vs vsa where vsa.col1=vs.col1) as seq
from vs
where col2 is null
order by col2 nulls last
) b
where a.rowid = b.rowid
where col2 is null;
SQL> select * from vs;
COL1 COL2
A 1
A 2
B 1
B 2
B 3
C 1
D 1
D 2
E 1
E 10
E 11
E 12
E 13
Might be helpful to someone else one day. Cheers everybody.