How to do this Update

Question

I need to add a 'serial_no' column to a table. The existing PK consists of non-consecutive values. The serial_no column which should follow the ordering of the PK and increased by exactly 1 for the next pk. How to write this UPDATE? I cannot use 'UPDATE t SET serial_no=ROWNUM' as the result is not guaranteed to be following the order of the PK.
pk, serial_no
10, 1
12, 2
14, 3
18, 4
....

Answer 1

here is one way to do :
SQL> select * from t;
PK SERIAL_NO
10
12
14
18
1 merge into t using (select pk, row_number() over (order by pk) rn from t) a
2 on (t.pk = a.pk)
3* when matched then update set t.serial_no = a.rn
SQL> /
4 rows merged.
SQL>
SQL> select * from t;
PK SERIAL_NO
10 1
12 2
14 3
18 4