如何比較SQL中的兩列？

Question

可以說我有兩個都有SSN變量的表，並且我想只顯示一個表，而不是兩個表。

正確的方法是什么？

Answer 1

這是一種方法：

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;

這適用於大多數數據庫，但不適用於MySQL。 以下內容幾乎可以在任何數據庫中工作：

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