Oracle query to match all values in the list among all rows in table

Question

I have a table structure as under:

ID    A     B
--   ---  -----
 1    a     a1
 2    b     b1
 3    a     a2
 4    c     a1
 5    b     b2

Answer 1

SELECT DISTINCT A      
FROM tablename   
WHERE B = 'a1'
OR B = 'a2'     

I think this should do the trick. Distinct is added so the value 'a' in A will only display once.