Unique value in a column

Hi,

I want to get the unique values in a column for a particular value in another column.

eg:

Column X Column Y

1234 A
1234 B
1234 C
3456 A
5678 A
7896 A
7896 B

Here I want to find the values in column X whose value in column Y is "A".
But if the same field has B or C it should not be selected.
e.g. in this case my list should yield 3456 and 5678 because they have only "A" in column Y and not "B" or "C".