Question

I have two tables, in one to many relationship (Quizzes, Comments): a Quiz can have multiple Comments

I need to display inside a page the last 5 comments for each quiz.

Is it possible to extract theses comments using one single query (if no, which is the best way to do it)? Right now I am executing a separate query for each quiz to extract the last 5 comments.

(my hope is to find a single HQL to allow me extracting theses comments)

ps. I am using hibernate/jpa/mysql

Answer 1

I wrote a complicated SQL that runs with MySQL 😉
the basic idee is:

• Order the comments and adding a rownum as rank. comments to same quizz have generated ranks with contiguous nummers
• join a max aggregation with the newest comment per quizz
• with this added information it’s possible to build a where clause to limit comments per quizz

Precondition is that a comment with a newer date has a higher id

Update: changed SQL. (realized some missing test cases in my small set of test data)

This part of the SQL is later used twice… you should create a View with that.

SELECT 
  @rownum:=@rownum+1 AS Rank,
  c.*
FROM _comments c, (SELECT @rownum:=0) r
ORDER BY  c.q_id, c.id

–

SELECT 
  ranked_c.*
FROM (
    SELECT 
      @rownum:=@rownum+1 AS Rank,
      c.*
    FROM _comments c, (SELECT @rownum:=0) r
    ORDER BY  c.q_id, c.id
  ) ranked_c
  INNER JOIN (
    SELECT 
      i.q_id,
      MAX(i.Rank) AS LastEntry_id
    FROM  (
      SELECT 
        @rownum:=@rownum+1 AS Rank,
        c.*
      FROM _comments c, (SELECT @rownum:=0) r
      ORDER BY c.q_id, c.id
    ) i
    GROUP BY i.q_id
  ) max_c ON ranked_c.q_id = max_c.q_id
WHERE max_c.LastEntry_id - ranked_c.Rank BETWEEN 0 AND 4 

alternative where clause: abs(max_c.LastEntry_id - ranked_c.Rank) < 5

–> Solution using a View:

CREATE OR REPLACE VIEW V_RankedComments AS (
  SELECT 
    @rownum:=@rownum+1 AS Rank,
    c.*
  FROM _comments c, (SELECT @rownum:=0) r
  ORDER BY c.q_id, c.id
)
SELECT 
  ranked_c.*
FROM V_RankedComments ranked_c
  INNER JOIN (
    SELECT 
      i.q_id,
      MAX(i.Rank) AS LastEntry_id
    FROM V_RankedComments i
    GROUP BY i.q_id
  ) max_c ON ranked_c.q_id = max_c.q_id
WHERE max_c.LastEntry_id - ranked_c.Rank BETWEEN 0 AND 4