Merge Update in duplicated rows oracle

Question

I have a table with repeated rows but with the empty group_id field:

id	reference1	group_id
1	AAAA
2	AAAA
3	AAAA
4	BBBB
5	BBBB
6	CCCC
7	CCCC
8	CCCC
9	CCCC
10	CCCC

I need to add a number to the group_id field as long as it is null and I use:

UPDATE journals_cv027_tab f1
   SET f1.group_id = journals_seq.nextval
 WHERE f1.group_id IS NULL
   AND EXISTS (SELECT reference1
                 FROM journals_cv027_tab c
                WHERE c.reference1 = f1.reference1);
COMMIT;    

This just added me a number for each record as follows:

id	reference1	group_id
1	AAAA	1
2	AAAA	2
3	AAAA	3
4	BBBB	4
5	BBBB	5
6	CCCC	6
7	CCCC	7
8	CCCC	8
9	CCCC	9
10	CCCC	10

But I need to assign a number for each group of repeated records for example:

id	reference1	group_id
1	AAAA	1
2	AAAA	1
3	AAAA	1
4	BBBB	2
5	BBBB	2
6	CCCC	3
7	CCCC	3
8	CCCC	3
9	CCCC	3
10	CCCC	3

Answer 1

I think no need a sequence based on your case, you rather can use DENSE_RANK() analytic function within a MERGE statment with MATCHED option, presumingly id is a primary key, which can easily be used within the matching condition of the subquery such as

MERGE INTO journals_cv027_tab j1 
     USING 
     (SELECT j.*, 
             DENSE_RANK() OVER (ORDER BY reference1) AS dr
        FROM journals_cv027_tab j) j2
          ON ( j2.id = j1.id )
      WHEN MATCHED THEN UPDATE SET j1.group_id = dr
     WHERE j1.group_id IS NULL

Demo

If you want to keep the value of the sequential number, then an option would be storing the max value for the group_id within a table which has one row and one column before truncating the table in order to use within the next update as <that value> + DENSE_RANK()...