merge into mytable t
using (SELECT ROWNUM rn, ID
FROM (
SELECT ID
FROM MYTABLE
WHERE PARENT = 1
ORDER BY SEQUENCE ASC
)) S
on (t.id=s.id)
when matched then update set
t.sequence=s.rn
If id is not unique, you can, for sure:
merge into mytable t
using (SELECT ROWNUM rn, rwd
FROM (
SELECT rowid rwd
FROM MYTABLE
WHERE PARENT = 1
ORDER BY SEQUENCE ASC
)) S
on (t.rowid=s.rwd)
when matched then update set
t.sequence=s.rn