过滤掉真正独特的行

Question

A | B
=====
1 | 2
3 | 4
2 | 1

如何过滤此表并仅获取唯一的行？这样的事情：

1 | 2
3 | 4

Answer 1

我想你想要这样的东西：

select a, b
from t
where a <= b
union all
select a, b
from t
where a > b and
      not exists (select 1 from t t2 where t2.a = t.b and t2.b = t.a);

注意：这假设您的数据中没有实际的重复项，而您只有一个保留一对，无论顺序如何。

Answer 2

使用不同的

select distinct least(a,b) as a, greatest(a,b) as b
from the_table;

或者，如果您的DBMS不支持least()和greatest()

select distinct 
         case when a < b then a else b end as a,
         case when a > b then a else b end as b
from the_table;

Answer 3

更简单的版本，省略了子查询并通过依靠UNION来处理实际的重复项：

SELECT a, b FROM test WHERE a <= b
UNION
SELECT b, a FROM test WHERE a > b;