Question

给出一个具有双向对的表，即A＆B，B＆A：

+-----------------------+
| Person_1 |   Person_2 |
+----------+------------+
| Nancy    |   John     |
| Nancy    |   Mary     |   
| John     |   Nancy    |
| Peter    |   Jane     |
| Jane     |   Peter    |
+----------+------------+

如何仅选择任何方向上的唯一对，例如：

+-----------------------+
| Person_1 |   Person_2 |
+----------+------------+
| Nancy    |   John     |
| Nancy    |   Mary     |
| Peter    |   Jane     |   
+----------+------------+

Answer 1

一种方法是：

select person_1, person_2
from t
where person_1 < person_2
union al
select person_1, person_2
from t
where person_1 > person_2 and
      not exists (select 1 from t t2 where t2.person_1 = t.person_2 and t2.person_2 = t.person_1);

这将保留对的原始顺序-因此返回的行在源数据中。如果那不重要：

select distinct least(person_1, person_2), greatest(person_1, person_2)
from t;

Answer 2

一种方法是使用具有exists条件的相关子查询：

select *
from mytable t
where 
    t1.person_1 < t.person_2 
    and exists (
        select 1
        from mytable t1
        where t1.person_1 = t.person_2 and t1.person_2 = t.person_1
)

双向对中的SQL唯一对

2 个答案: