You could use a combination of UNION and EXCEPT to get the unmatching records.
Something like this:
Alternatively, you could use subqueries to achieve the same result:
<pre>select col_a from table_1
where col_a not in (select col_a from table_2)
select col_a from table_2
where col_a not in (select col_a from table_1)
Hope this helps.