Last 5 comments for x articles query with mysql

Question

I have 3 tables:

-categories (-categories not important)

-articles

-comments

---

example:

-articles I have 10000+ articles

-comments I have 10000+ comments for random -articles

---

-I'm trying to show "last 5 comments"

OF articles.article_id s= in(1, 2, 3, 4, 5)

WITH order by comments.comment_id desc

WITH categories.category_id,categories.category_name

-SQL-QUERY-1:

select 
comments.comment_id,comments.comment_title,comments.article_id, 
articles.article_id,articles.article_title,articles.category_id, 
categories.category_id,categories.category_name from comments 
left join (articles,categories) on (comments.article_id=articles.article_id and articles.category_id=categories.category_id) 
where (comments.article_id in(1,2,3,4,5)) 
ORDER BY `comments`.`comment_id`  DESC limit 0 , 5

Please focus to comments_id and article_id columns

• sql-1: http://sqlfiddle.com/#!2/3ab6c1/1

As you see;

• comments.comment_id desc order +OK

109

108

107

106

105

comments.article_id s +OK

1 - 2 - 3 - 4 - 5

but result list shows **2 comments of every article**

-I need to "group" articles.article_id s

so I add " group by comments.article_id"

-SQL-QUERY-2:

select 
comments.comment_id,comments.comment_title,comments.article_id, 
articles.article_id,articles.article_title,articles.category_id, 
categories.category_id,categories.category_name from comments 
left join (articles,categories) on (comments.article_id=articles.article_id and articles.category_id=categories.category_id) 
where (comments.article_id in(1,2,3,4,5)) 
/*for duplicate comments*/ GROUP by comments.article_id /*for duplicate comments*/
ORDER BY `comments`.`comment_id`  DESC limit 0 , 5

Please focus to comments_id and article_id columns

-sql-2: http://sqlfiddle.com/#!2/3ab6c1/2

it's ok, comments.article_id s grouped BUT "comments.comment_id desc" order is broken;

108

106

104

102

100

(must be 109 106 104 102 100)

---

I'm not an expert of mysql(I lost more than 3-4 hours)

---

-I need to show: "last(desc) 5 comments" in every category listing pages.

-I need to show: "last(desc) "1" comment for every article in(1,2,3,4,5 article_id s)"

---

(distinct and max shows same results 108 106 104 102 100

If I add group by comments.article_id and remove order by comments.comment_id desc, the list shows FIRST 1 comment of article -- I need to show LAST 1 comment of article)

Can any sql expert help me for this sql query ?

Answer 1

Assuming you want to show the last 5 comments of 5 specific articles, one comment per article:

select     c.comment_id, c.comment_title, c.article_id, 
           a.article_id, a.article_title, a.category_id, cats.category_name
from       articles      a
inner join categories cats on (a.category_id = cats.category_id)
inner join (select   article_id, max(comment_id) comment_id, comment_title
            from     comments co
            where    co.article_id in (1, 2, 3, 4, 5)
            group by co.article_id) c
            on (a.article_id = c.article_id)
order by    c.comment_id desc

http://sqlfiddle.com/#!2/3ab6c1/27

Answer 2

Does this help at all...

 SELECT x.*
      , COUNT(*) rank
   FROM comments x
   JOIN comments y 
     ON y.article_id = x.article_id
    AND y.comment_id <= x.comment_id
  GROUP
     BY article_id
      , comment_id;