这是一种方法：

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;

这适用于大多数数据库，但不适用于MySQL。以下内容几乎可以在任何数据库中工作：

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