PostgreSQL的SQL語句：無法獲取所需的最新/最近的評論，需要顯示了最新評論

Question

\ d評論

        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) 

對於這一說法

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

獨特posts.title我

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) 

但我需要這樣

結果0

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

我該如何重寫sql語句來獲得這三行作爲結果？

\ d柱

        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) 

與來自三行我需要從職位表中的posts.title三個ID。 我如何編寫sql語句來獲取post.titt與comments.post_id = 5或8或2？

Answer 1

編輯：修改基於評論的答案。

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;