Querying Two Inter-related Field In A Table

Hi,

Need some help. Here's the problem:-

I've two fields in a table (A & B)

A B
1 Null
2 Null
3 1
4 2
5 3
6 5
7 Null
8 4
9 6

The fields are somehow related where values of B (unique) is taken from A(unique).
I trying to do an SQL such that if I input a single value A=9 or 6 or 5 or 3 or 1, the following should be the output:

A B
9 6
6 5
5 3
3 1
1 Null

Or in another case where I input a single value A=8 or 4 or 2
the following should be the output:

A B
8 4
4 2
2 Null

How can I do that? Many thanks in advance!! :D