Query to retrieve an intersection of sorts

[Saeven]

Hello,

I need a hand with the following, given that I have one table, with two columns containing the following data:

letter | value

A 1
A 2
A 3
B 1
B 3

I need to return letter-wise sets, where the whole set is returned if BOTH 1 and 2 are present, otherwise, the set excluding 1 and 2 is returned. (1 and 2 are returned only if they are both present).

So my query for A would return:
1,2,3

my query for B would yield:
3

Any help appreciated.
Thanks in advance.
Saeven