如何 select COUNT(*) 的最大值 (SQL postgresql)

Question

我从 NOSQL 切换到 SQL 但我找不到如何 select count(*)

的最大值

我创建了用户 table、帖子和评论。我想 select 发帖数和评论数最多的前 10 名用户

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

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 2

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

由于您需要获得 TOP 10 个具有最多 post 的用户，因此当用户具有相同的 post 计数并且应该使用 LIMIT 10 的传统方法时会出现问题视为同等级。例如：4个人有5个post，3个人有6个post..

为了解决上面相同的 post 计数问题，我们将选择 RANK query，这会将具有相同 post 计数的人视为相同等级。

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;

破碎成小块：

1.Get post 每个用户计数

首先，我们得到每个用户的 post 计数，这需要 COUNT 函数，并且 GROUP BY author_id

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

2。计算 post count

的排名

使用RANK()函数计算post的排名。排名位置由total_posts

的最高人数决定

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 table 与 posting_rank_stats table 连接起来以获得结果。由于我们只想进入最多 post 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 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;

如何 select COUNT(*) 的最大值 (SQL postgresql)

how to select the max of COUNT(*) (SQL postgresql)

sql

node.js

postgresql

node-postgres

1.Get post 每个用户计数

2。计算 post count

3。总结