相关疑难解决方法(0)

正如标题所示,我想选择用a组成的每组行的第一行GROUP BY.

具体来说,如果我有一个purchases看起来像这样的表:

SELECT * FROM purchases;

我的输出:

id | customer | total
---+----------+------
 1 | Joe      | 5
 2 | Sally    | 3
 3 | Joe      | 2
 4 | Sally    | 1

我想查询每个产品id的最大购买量(total)customer.像这样的东西:

SELECT FIRST(id), customer, FIRST(total)
FROM  purchases
GROUP BY customer
ORDER BY total DESC;

预期产出:

FIRST(id) | customer | FIRST(total)
----------+----------+-------------
        1 | Joe      | 5
        2 | Sally    | 3

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';

我得到了那个,但我也想按照分组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 …