Postgresql 删除重复的反向对

Question

我有这个table:

 origin     destination

new york      seattle
new york      chicago
new york      portland
seattle       new york
seattle       chicago
chicago       new york

我必须构建一个图表，所以我需要删除所有重复的反向对：

origin      destination oneway

new york      seattle     0
new york      chicago     0
new york      portland    1
seattle       chicago     1

我已经读过这个post： SQL -- Remove duplicate pairs 但它对我没有用，因为我有字符串归档。

谢谢

Answer 1

一种方法使用聚合：

select origin, destination,
       (case when exists (select 1
                          from t t2
                          where t2.origin = t.destination and t2.destination = t.origin
                         )
             then 0 else 1
        end) as one_way
from t
where origin < destination
union all
select origin, destination, 1
from t
where origin > destination;

另一种方法使用 window 函数：

select origin, destination, (cnt = 1)::int as one_way
from (select t.*,
             count(*) over (partition by least(origin, destination), greatest(origin, destination)) as cnt
      from t
     ) t
where origin < destination or
      (origin > destination and cnt = 1);

Answer 2

row_number 和 count 使用 least 和 greatest 的一个选项。

select origin,dest,case when cnt_per_pair=1 then 1 else 0 end as one_way
from (select t.*,row_number() over(partition by least(origin,dest),greatest(origin,dest) 
                                   order by dest) as rnum,
      count(*) over(partition by least(origin,dest),greatest(origin,dest)) as cnt_per_pair
      from tbl t
     ) t
where rnum=1

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