幫助SQL查詢

Question

我有以下表博客（ID）和帖子（ID，blog_id，comment_count） 我需要從不同的博客中選擇五個最多評論的職位。使用函數很容易，但是可以使用某種基本的SQL嗎？

下面是在SQL Server方言

select top 5 top_post.* from Blogs b 
cross apply 
    (select top 1 * from Posts p 
    where p.blog_id = b.id 
    order by p.comment_count) top_post 
order by top_post.comment_count 

Answer 1

SELECT b.*, c.num_comments 
FROM 
(
    SELECT TOP 5 blog_id, SUM(comment_count) as num_comments 
    FROM posts GROUP BY blog_id ORDER BY SUM(comment_count) DESC 
)c 
INNER JOIN Blogs b ON (b.id = c.blog_id) 

UPDATE查詢希望這是你所需要的。雖然它不是很快。

SELECT b.*, c.comment_count 
FROM 
(SELECT blog_id, comment_count , 
ROW_NUMBER() OVER(PARTITION by blog_id ORDER BY comment_count DESC) as rnum 
FROM posts 
)c 
INNER JOIN Blogs b ON (b.id = c.blog_id) 
WHERE c.rnum <=5; 

Answer 2

據我所知，是實現這實際上是由主要的DBMS支持Select Top沒有標準的方式。

查看comparison of SQL dialects