选择多个列，但在postgres中只有一个列

Question

我在postgres中有一个简单的表：

select remoteaddr,
count (remoteaddr)
from domain_visitors
group by remoteaddr
having count (remoteaddr) > 500

我使用以下内容进行了调整：

remoteaddr

如何选择其他列，仍然只按{{1}}分组？

Answer 1

选项1：您可以使用array_agg()函数将其他列值连接到分组列表中：

SELECT 
   remoteaddr, 
   array_agg(DISTINCT username) AS unique_users, 
   array_agg(username) AS repeated_users, 
   count(remoteaddr) as remote_count
FROM domain_visitors
GROUP BY remoteaddr;

见this SQL Fiddle。此查询将返回如下所示的内容：

+----------------+---------------------------------+-----------------------------------------------------------------------------------------------------+--------------+
|   remoteaddr   |          unique_users           |                                           repeated_users                                            | remote_count |
+----------------+---------------------------------+-----------------------------------------------------------------------------------------------------+--------------+
|  142.4.218.156 | anotheruser,user9688766,vistor1 | user9688766,anotheruser,vistor1,vistor1,vistor1,vistor1,vistor1,anotheruser,anotheruser,anotheruser |           10 |
|  158.69.26.144 | anotheruser,user9688766         | anotheruser,user9688766,user9688766,user9688766,user9688766                                         |            5 |
| 167.114.209.28 | vistor1                         | vistor1                                                                                             |            1 |
+----------------+---------------------------------+-----------------------------------------------------------------------------------------------------+--------------+

选项2：您可以将第一个查询放在common table expression（又称“WITH”子句）中，并将其与原始表连接，如下所示：

WITH grouped_addr AS (
  SELECT remoteaddr, count(remoteaddr) AS remote_count
  FROM domain_visitors
  GROUP BY remoteaddr
)

SELECT ga.remoteaddr, dv.username, ga.remote_count
FROM grouped_addr ga
INNER JOIN domain_visitors dv
ON ga.remoteaddr = dv.remoteaddr
WHERE remote_count > 500;

这是SQL Fiddle。

请记住，这将返回任何其他列的重复结果（在此示例中为username）。这不是通常你想要的。请注意Fiddles中的每个SELECT示例，看看哪个最适合您的目的。