Postgresql:选择distinct -列组合

Question

我在PostgreSQL中有这样的查询：

select p1.id, p2.id, p1.title, p2.title 
from publication "p1", publication "p2"
where p1.title = p2.title and p1.id <> p2.id

问题是它返回的数据比我需要的多：

id    id   title          title  
3456  5678 Ulysses        Ulysses  
5678  3456 Ulysses        Ulysses  
234,  345  Das Kapital    Das Kapital  
345   234  Das Kapital    Das Kapital 

我只需要第一行和第三行，或者第二行和第四行。

Answer 1

select p1.id, p2.id
 , p1.title, p2.title
from publication p1
    , publication p2
where p1.title = p2.title
  and p1.id < p2.id -- tie breaker
  ;

或者使用更时髦的JOIN语法：

SELECT p1.id, p2.id
 , p1.title, p2.title
FROM publication p1
JOIN publication p2 ON p1.title = p2.title
                   AND p1.id < p2.id -- tie breaker
  ;

Answer 2

我有一个简单的想法来实现你的场景。

select p1.id, p2.id, p1.title, p2.title , sum(p1.id + p2.id) as temp
from publication "p1", publication "p2" group by temp

Answer 3

select DISTINCT p1.id, p2.id, p1.title, p2.title 
from publication "p1", publication "p2"
where p1.title = p2.title