Incrementing a "Private" Integer Space

[David T. Ashley]

I have a table with two integer fields (call them p and q).

When I insert a record with a known p, I want to choose q to be one larger
than the largest q with that p.

What is the best and most efficient way to do this?

For example, let's say the table contains (p,q):

1,1
1,2
1,3
2,1
2,2
2,3
2.4
2,5
3,1
3,2

If I insert a new record with p=2, I would want to choose q to be 6.  But if
I insert a record with p=3, I would want to choose q to be 3.

Is there any alternative to locking the table, querying for max q with the
desired p, then inserting?

Thanks.