如何在PostgreSQL查询中排序不同的元组

Question

我正在尝试在Postgres中提交一个只返回不同元组的查询。 在我的示例查询中，我不希望对于cluster_id / feed_id组合多次存在条目的重复条目。 如果我做一个简单的事：

select distinct on (cluster_info.cluster_id, feed_id) 
   cluster_info.cluster_id, num_docs, feed_id, url_time 
   from url_info 
   join cluster_info on (cluster_info.cluster_id = url_info.cluster_id) 
   where feed_id in (select pot_seeder from potentials) 
   and num_docs > 5 and url_time > '2012-04-16';

我得到了，但我也想根据num_docs进行num_docs 。 所以，当我做以下事情时：

select distinct on (cluster_info.cluster_id, feed_id) 
   cluster_info.cluster_id, num_docs, feed_id, url_time 
   from url_info join cluster_info 
   on (cluster_info.cluster_id = url_info.cluster_id) 
   where feed_id in (select pot_seeder from potentials) 
   and num_docs > 5 and url_time > '2012-04-16' 
   order by num_docs desc;

我收到以下错误：

ERROR:  SELECT DISTINCT ON expressions must match initial ORDER BY expressions
LINE 1: select distinct on (cluster_info.cluster_id, feed_id) cluste...

我想我理解为什么我会收到错误（除非我以某种方式明确描述该组，否则不能通过元组进行分组）但是我该怎么做？ 或者，如果我对错误的解释不正确，有没有办法实现我的初始目标？

Answer 1

最左边的ORDER BY项不能与DISTINCT子句的项不一致。 我引用了关于DISTINCT的手册 ：

DISTINCT ON表达式必须与最左边的ORDER BY表达式匹配。 ORDER BY子句通常包含其他表达式，用于确定每个DISTINCT ON组中行的所需优先级。

尝试：

SELECT *
FROM  (
    SELECT DISTINCT ON (c.cluster_id, feed_id) 
           c.cluster_id, num_docs, feed_id, url_time 
    FROM   url_info u
    JOIN   cluster_info c ON (c.cluster_id = u.cluster_id) 
    WHERE  feed_id IN (SELECT pot_seeder FROM potentials) 
    AND    num_docs > 5
    AND    url_time > '2012-04-16'
    ORDER  BY c.cluster_id, feed_id, num_docs, url_time
           -- first columns match DISTINCT
           -- the rest to pick certain values for dupes
           -- or did you want to pick random values for dupes?
    ) x
ORDER  BY num_docs DESC;

或者使用GROUP BY ：

SELECT c.cluster_id
     , num_docs
     , feed_id
     , url_time 
FROM   url_info u
JOIN   cluster_info c ON (c.cluster_id = u.cluster_id) 
WHERE  feed_id IN (SELECT pot_seeder FROM potentials) 
AND    num_docs > 5
AND    url_time > '2012-04-16'
GROUP  BY c.cluster_id, feed_id 
ORDER  BY num_docs DESC;

如果c.cluster_id, feed_id是所有（在本例中都是）表中包含SELECT列表中的列的主键列，那么这只适用于PostgreSQL 9.1或更高版本。

否则，您需要GROUP BY其余列或聚合或提供更多信息。