選擇用戶沒有評論過的博客文章

Question

一篇文章有​​很多評論。評論（內容，author_id）由用戶創作。

如何檢索給定用戶（author_id = 1）未發表評論的博客文章？

謝謝。

Answer 1

SELECT p.* 
    FROM post p 
    WHERE NOT EXISTS(SELECT 1 
         FROM comment c 
         WHERE c.post_id = p.post_id 
          AND c.author_id = 1) 

你也可以用左這樣做JOIN：

SELECT p.* 
    FROM post p 
     LEFT JOIN comment c 
      ON p.post_id = c.post_id 
       AND c.author_id = 1 
    WHERE c.post_id IS NULL 

Answer 2

在常規的「OLE SQL，這應該是這樣的：

SELECT post_id 
FROM posts p 
WHERE NOT EXISTS (
    SELECT 1 
    FROM comments c 
    WHERE p.post_id = c.post_id 
     AND c.author_id = 1 
) 

我不知道你怎麼樣用Ruby on Rails/ActiveRecord語法來做到這一點。