GROUP BY和COUNT在PostgreSQL中 [英] GROUP BY and COUNT in PostgreSQL

问题描述

查询：

SELECT COUNT(*) as count_all, 
       posts.id as post_id 
FROM posts 
  INNER JOIN votes ON votes.post_id = posts.id 
GROUP BY posts.id;

在Postgresql中返回 n p>

Returns n records in Postgresql:

 count_all | post_id
-----------+---------
 1         | 6
 3         | 4
 3         | 5
 3         | 1
 1         | 9
 1         | 10
(6 rows)

我只想检索返回的记录数： code> 6 。

我使用子查询来实现我想要的，但这似乎不是最佳的：

I used a subquery to achieve what I want, but this doesn't seem optimum:

SELECT COUNT(*) FROM (
    SELECT COUNT(*) as count_all, posts.id as post_id 
    FROM posts 
    INNER JOIN votes ON votes.post_id = posts.id 
    GROUP BY posts.id
) as x;

我如何在PostgreSQL中获得此上下文中的记录数？

How would I get the number of records in this context right in PostgreSQL?

推荐答案

我想你只需要 COUNT（DISTINCT post_id）FROM votes 。

请参阅 http中的4.2.7。汇总表达式部分：//www.postgresql.org/docs/current/static/sql-expressions.html 。

编辑：根据Erwin的评论更正了我的粗心错误。

Corrected my careless mistake per Erwin's comment.

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