postgresql sql语句：无法获得所需的最新/最近评论，需要显示带有最新评论的不同posts.title

Question

\d cmets

                                   Table "public.comments"
   Column   |          Type          |                       Modifiers                       
------------+------------------------+-------------------------------------------------------
 id         | integer                | not null default nextval('comments_id_seq'::regclass)
 post_id    | integer                | not null
 name       | character varying(255) | not null
 email      | character varying(255) | not null
 content    | character varying(500) | not null
 created_at | date                   | 
 updated_at | date                   | 
Indexes:
    "comments_pkey" PRIMARY KEY, btree (id)

为此声明

select post_id,created_at from comments order by created_at limit 5;

我明白了

 post_id | created_at 
---------+------------
       5 | 2011-07-11
       5 | 2011-07-11
       5 | 2011-07-11
       8 | 2011-07-11
       2 | 2011-07-17
(5 rows)

但我需要这样的结果

 post_id | created_at 
---------+------------
       5 | 2011-07-11
       8 | 2011-07-11
       2 | 2011-07-17
(3 rows)

如何重写 sql 语句以得到这三行作为结果？

\d 个帖子

                                   Table "public.posts"
   Column    |          Type          |                     Modifiers                      
-------------+------------------------+----------------------------------------------------
 id          | integer                | not null default nextval('posts_id_seq'::regclass)
 title       | character varying(100) | not null
 content     | character varying(500) | not null
 created_at  | date                   | 
 updated_at  | date                   | 
 tags        | character varying(55)  | not null default '50'::character varying
 category_id | integer                | not null default 1
Indexes:
    "posts_pkey" PRIMARY KEY, btree (id)

使用这三行中的三个 id，我需要从 posts 表中获取 posts.title。 如何编写 sql 语句以获取带有 cmets.post_id = 5 或 8 或 2 的 posts.title ？

Answer 1

编辑：根据评论修改答案。

 SELECT p.title, q.LatestCommentDate
     FROM (SELECT c.post_id, MAX(c.created_at) AS LatestCommentDate
               FROM comment c
               GROUP BY c.post_id) q
         INNER JOIN posts p
             ON q.post_id = p.id;