select key1,key2,count(*)
from table
group by key1,key2
having count(*) > 1
order by 3 desc;

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确定重复项的查询的关键部分是having count(*) > 1。
在下面的链接中有一大堆整洁的技巧，包括一些删除重复的例子：http://postgres.cz/wiki/PostgreSQL_SQL_Tricks

假设您只想删除重复项并保留原始项，则接受的答案是不准确的--它也会删除原始项，只保留从一开始就有一个条目的记录。

SELECT * FROM tblname WHERE ctid IN 
(SELECT ctid FROM 
  (SELECT ctid, ROW_NUMBER() OVER 
    (partition BY col1, col2, col3 ORDER BY ctid) AS rnum 
  FROM tblname) t
WHERE t.rnum > 1);

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https://wiki.postgresql.org/wiki/Deleting_duplicates

select distinct a.*
from tbl a
inner join tbl b on (
a.id <> b.id
and b.col1 = a.col1
and b.col2 = a.col2
)
order by a.col1 asc

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