Come faccio a confrontare due colonne in SQL?

Ecco un modo:

select coalesce(t1.ssn, t2.ssn)
from t1 full outer join
     t2
     on t1.ssn = t2.ssn
where t1.ssn is null or t2.ssn is null;

Funziona nella maggior parte dei database, ma non in MySQL. Quanto segue dovrebbe funzionare praticamente in qualsiasi database:

select ssn
from ((select ssn, 't1' as which
       from t1
      ) union all
      (select ssn, 't2' as which
       from t2
      )
     ) t
group by ssn
having count(distinct which) = 1