遵循SQL语法可以在检查重复行的时候提供更好的性能。

SELECT id, count(id)
FROM table1
GROUP BY id
HAVING count(id) > 1

在您的情况下，由于限制，您需要删除重复的记录。

查找重复的行 根据created_at日期组织它们——在本例中，我保留了最老的日期 使用USING删除记录以过滤正确的行

WITH duplicated AS ( 
    SELECT id,
        count(*) 
    FROM products 
    GROUP BY id 
    HAVING count(*) > 1), 
ordered AS ( 
    SELECT p.id, 
        created_at, 
        rank() OVER (partition BY p.id ORDER BY p.created_at) AS rnk 
    FROM products o 
    JOIN     duplicated d ON d.id = p.id ), 
products_to_delete AS ( 
    SELECT id, 
        created_at 
    FROM   ordered 
    WHERE  rnk = 2
) 
DELETE 
FROM products 
USING products_to_delete 
WHERE products.id = products_to_delete.id 
    AND products.created_at = products_to_delete.created_at;

遵循SQL语法可以在检查重复行的时候提供更好的性能。

SELECT id, count(id)
FROM table1
GROUP BY id
HAVING count(id) > 1

受到Sandro Wiggers的启发，我做了一些类似的事情

WITH ordered AS ( 
  SELECT id,year, user_id, sid, cid,
    rank() OVER (PARTITION BY year, user_id, sid, cid ORDER BY id) AS rnk 
  FROM user_links 
), 
to_delete AS ( 
  SELECT id
  FROM   ordered 
  WHERE  rnk > 1
) 
DELETE 
FROM user_links
USING to_delete 
WHERE user_link.id = to_delete.id;

如果你想测试它，稍微改变一下:

WITH ordered AS ( 
  SELECT id,year, user_id, sid, cid,
    rank() OVER (PARTITION BY year, user_id, sid, cid ORDER BY id) AS rnk 
  FROM user_links 
), 
to_delete AS ( 
  SELECT id,year,user_id,sid, cid
  FROM   ordered 
  WHERE  rnk > 1
) 
SELECT * FROM to_delete;

这将给出将要删除的内容的概述(在运行删除时，在to_delete查询中保留year,user_id,sid,cid是没有问题的，但随后它们就不需要了)

为了方便起见，我假设您希望仅对列year应用唯一约束，并且主键是名为id的列。

为了找到重复的值，您应该运行，

SELECT year, COUNT(id)
FROM YOUR_TABLE
GROUP BY year
HAVING COUNT(id) > 1
ORDER BY COUNT(id);

使用上面的sql语句，您将得到一个包含表中所有重复年份的表。为了删除除最新重复条目外的所有重复条目，您应该使用上面的sql语句。

DELETE
FROM YOUR_TABLE A USING YOUR_TABLE_AGAIN B
WHERE A.year=B.year AND A.id<B.id;

从“用PostgreSQL查找重复行”这里有一个聪明的解决方案:

select * from (
  SELECT id,
  ROW_NUMBER() OVER(PARTITION BY column1, column2 ORDER BY id asc) AS Row
  FROM tbl
) dups
where 
dups.Row > 1