If I interpret your question correctly:

*"Pick all rows where `a` has one of two given values, and `b` only exists in combination with one of them."*

You can use any of the standard techniques laid out in the referenced answer.

- http://dba.stackexchange.com/questions/16650/help-with-this-select-in-the-same-table/16651#16651

Just restrict your base table to the two given `a`. For instance

    SELECT *
    FROM   tbl t1
    WHERE  a IN (1,2)
    AND    NOT EXISTS (
       SELECT 1
       FROM   tbl t2
       WHERE  a IN (1,2)
       AND    t2.b =  t1.b
       AND    t2.a <> t1.a
       );

[**SQL Fiddle**][1]

This also works with duplicates on `(a,b)`. Multiple identical rows would be returned.


  [1]: http://sqlfiddle.com/#!15/efbc3/1