组合两个列不管它们的顺序 - Mysql

Question

我有这张表在我的网站上安排邮件

mail_id   from   to    mail
   1        a     b    ....
   2        a     b    ....
   3        b     a    ....
   4        a     c    ....
   5        a     c    ....

现在，我想选择一个独特的from-to或to-from对。与他们的位置无关。这是期望的结果

from  to
 a    b
 a    c

我正在使用GROUP BY来进行列，但它给了我a-b和b-a

select * from mail group by from, to

Answer 1

   SELECT LEAST(FROM,TO),GREATEST(FROM, TO)
     from MAIL
     GROUP BY LEAST(FROM,TO),GREATEST(FROM, TO)

可能会这样做。

Answer 2

select * from mail 
where from<to
group by from, to