How do I compare two columns in SQL?

Question

Lets say I have a two table both with a SSN variable, and I want to show the ones that are in only one table, not both.

What is the correct way to do this?

Answer 1

Here is one way:

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;

This works in most databases, but not in MySQL. The following should work in pretty much any 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