Sequence Numbering in SQL Server

Question

I have an table (always ordered by ID ascending) with 5 records as such:

ID  Sequence
1   1
2   2
3   3
4   4
8   3
9   3

And the desired output is:

ID  Sequence
1   1
2   2
3   3
4   6
8   4
9   5

Answer 1

Looks like 3 direct updates to me, no point over complicating it:

UPDATE table SET sequence = 6 WHERE id = 4 
UPDATE table SET sequence = 4 WHERE id = 8
UPDATE table SET sequence = 5 WHERE id = 9

Answer 2

If you want to do this in one step:

update t
    set sequence = v.sequence
    from t join
         (values (4, 6), (8, 4), (9, 5)
         ) v(id, sequence)
         on t.id = v.id;

If you have to do many of these updates, then separate calls to update incur extra overhead.