SQL去重问题（应该是比较高级的去重？）

Question

原题：
List the names of all the pairs of U.S. counties that touch each other only in a point (or multiple points, but not lines). Make sure to list each pair only once, and sort results alphabetically by the name of the first county in a pair and then by the name of the second county.

问题：
如何保证每一对只显示一次？（make sure to list each pair only once）。

举例：
返回的记录是：
1 A B
2 A C
3 A D
4 B A

我们视 1 和 4 是重复的， 如何去除

Answer 1

SELECT (CASE WHEN 第一列<第二列 THEN 第一列+第二列 ELSE 第二列+第一列 END) as AB from table group by AB