Find all values not present for other values of other column
If your pairs are unique, as in the example you gave, you want the values of b that appear only once, and this would be a solution:

SELECT a,b
FROM tbl1
WHERE b in (
  SELECT b
  FROM tbl1
  GROUP BY b
  HAVING (count(b)) = 1
  )


You can see it in this fiddle http://sqlfiddle.com/#!15/c82d3/3