PostgreSQL中的GROUP BY和COUNT

Question

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;

返回nPostgresql中的记录：

 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：6。

我使用了一个子查询来实现我想要的，但这似乎不是最佳的：

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;

如何在PostgreSQL中获得这种上下文中的记录数？

Answer 1

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

请参阅http://www.postgresql.org/docs/current/static/sql-
expressions.html中的
“ 4.2.7。聚合表达式”部分。

编辑：根据Erwin的评论更正了我的粗心大意的错误。