如何选择 COUNT(*) 的最大值（SQL postgresql）

Question

我从 NOSQL 切换到 SQL，但找不到如何选择最大计数 (*)

我创建了用户表、帖子和 cmets。 我想选择帖子最多和 cmets 最多的 TOP 10 用户

SELECT 
  fullname, 
   (SELECT COUNT(*)
   FROM posts WHERE posts.author_id = users.id) 
    AS total_posts,
  (SELECT COUNT(*)
    FROM comments WHERE comments.author_id = users.id)
    AS total_comments 
  FROM users

Answer 1

首先，使用SQL数据库需要在关联表之间定义JOIN语句来获取引用的关系数据。

由于您需要获得大多数帖子的TOP 10 用户，因此当用户具有相同的帖子数并应被视为相同排名时，使用LIMIT 10 的传统方法会出现问题。 E.g: 4人发5帖，3人发6帖..

为了解决上面相同的帖子数问题，我们将选择RANK query，这会将帖子数相同的人视为相同的排名。

SELECT users.fullname, posting_rank_stats.total_posts, posting_rank_stats.posting_rank
FROM users
INNER JOIN
(
    SELECT author_id, total_posts, RANK() OVER (ORDER BY post_author_count.total_posts DESC) AS posting_rank
    FROM
    (
        SELECT COUNT(*) AS total_posts, author_id
        FROM posts
        GROUP BY author_id
    ) post_author_count
) posting_rank_stats
ON users.id = posting_rank_stats.author_id
-- we only want to search for top 10
WHERE posting_rank_stats.posting_rank <= 10;

碎成小块：

1.获取每个用户的帖子数

首先，我们获取每个用户的帖子数，这需要COUNT 函数和GROUP BY author_id

SELECT COUNT(*) AS total_posts, author_id
FROM posts
GROUP BY author_id

2。计算帖子数排名

使用RANK() 函数计算帖子的排名。排名位置由total_posts的最高数量决定

SELECT author_id, total_posts, RANK() OVER (ORDER BY post_author_count.total_posts DESC) AS posting_rank
FROM
(
    SELECT COUNT(*) AS total_posts, author_id
    FROM posts
    GROUP BY author_id
) post_author_count

3。总结

最后一步是将users 表与posting_rank_stats 表连接以获得结果。由于我们只想进入帖子最多的 TOP 10 用户，因此您需要添加条件 posting_rank_stats.posting_rank <= 10 在WHERE 子句中

应用相同的概念也可以获得评论排名。

这里是db fiddle I created, combine both posting rank and commenting rank

SELECT users.id , users.fullname, posting_rank_stats.posting_rank , posting_rank_stats.total_posts, commenting_rank_stats.comment_rank, commenting_rank_stats.total_comments
FROM users

LEFT JOIN
(
    SELECT author_id, total_posts, RANK() OVER (ORDER BY post_author_count.total_posts DESC) AS posting_rank
    FROM
    (
        SELECT COUNT(*) AS total_posts, author_id
        FROM posts
        GROUP BY author_id
    ) post_author_count 
) posting_rank_stats ON users.id = posting_rank_stats.author_id

LEFT JOIN
(
    SELECT author_id, total_comments, RANK() OVER (ORDER BY comment_author_count.total_comments DESC) AS comment_rank
    FROM 
    (
        SELECT COUNT(*) AS total_comments, author_id
        FROM comments
        GROUP BY author_id
    ) comment_author_count
) commenting_rank_stats ON users.id = commenting_rank_stats.author_id;

id	fullname	posting_rank	total_posts	comment_rank	total_comments
1	Jackson	1	3	5	1
2	Marry	4	1	1	5
3	Josh	4	1	2	3
4	Harley	1	3	4	2
5	Gordon	4	1	5	1
6	Barney	4	1	2	3
7	Gman	4	1
8	Stephan	3	2
9	Lucy
10	Jordan
11	Bill
12	Rosh
13	Lee

---

Answer 2

您可以将ORDER BY 与LIMIT 结合使用。例如：

select *
from (
  -- your query here
) x
order by total_posts desc
limit 10

Answer 3

select * from (
   select u.fullname,count(p.posts) as totalposts, count(c.comments) as totalcomments
   from users u join posts p
   on u.id=p.author_id 
   join comment c on u.id=c.comments
   group by 1
   ORDER BY count(p.posts),count(c.comments) desc )  k
limit 10;