Besoin d'aide pour placer LIMIT, OFFSET dans cette requête MySQL

Question

Cette requête renvoie les questions/réponses du forum et leurs commentaires imbriqués (similaire au paradigme StackOverflow).

SELECT forum_qa.*, 
      user_profiles.*, 
      c.*, 
      n.pid, 
      v.*, 
      Ifnull(n.ans_count, 0) AS ans_count 
    FROM forum_qa 
      JOIN user_profiles 
      ON user_id = forum_qa_author_id 
      LEFT JOIN (SELECT * 
         FROM votes) AS v 
      ON forum_qa_id = v.forum_qa_id_fk 
      LEFT JOIN (SELECT forum_cm_id, 
          forum_cm_author_id, 
          forum_qa_id_fk, 
          forum_cm_text, 
          forum_cm_timestamp, 
          forum_cm_flag, 
          first_name AS forum_cm_first_name, 
          last_name AS forum_cm_last_name, 
          facebook_id AS forum_cm_fb_id, 
          picture  AS forum_cm_picture, 
          moderator AS forum_cm_moderator 
         FROM forum_cm 
          JOIN user_profiles 
           ON user_id = forum_cm_author_id) AS c 
      ON forum_qa_id = c.forum_qa_id_fk 
      LEFT JOIN (SELECT forum_qa_parent_id AS pid, 
          COUNT(*)   AS ans_count 
         FROM forum_qa 
         WHERE forum_qa_parent_id IS NOT NULL 
         GROUP BY forum_qa_parent_id) AS n 
      ON forum_qa_id = n.pid 
    WHERE forum_qa_id LIKE "%" 
      AND forum_qa_parent_id IS NULL 
    ORDER BY forum_qa_timestamp DESC 
      LIMIT 0,3 

Je suis en train de paginer les résultats et je frappais le problème suivant:

En plaçant LIMIT à la fin de la requête, je finissent par limiter le nombre de au total lignes, non le nombre de questions/réponses.

Ex, LIMIT 0,3 dans la dernière ligne me donne (qa: questions/réponses; cm: un commentaire):

forum_qa_id qa_text forum_cm_id cm_text 
1    asd 
2    wer  4    this is a comment 
2    wer  5    this is another comment 

au lieu de

forum_qa_id qa_text forum_cm_id cm_text 
1    asd 
2    wer  4    this is a comment 
2    wer  5    this is another comment 
3    zxc  
3    zxc  7    yet another comment 

Toute suggestion comment modifier ma requête dans ordre d'avoir LIMIT 0,3 retourner pas 3 lignes mais 3 questions quel que soit le nombre de commentaires imbriqués est a?

changements avec @ la suggestion de Michael

SELECT qa.*, 
      user_profiles.*, 
      c.*, 
      n.pid, 
      v.*, 
      Ifnull(n.ans_count, 0) AS ans_count 
    FROM (SELECT * FROM forum_qa LIMIT 0, 3) qa 
      JOIN user_profiles 
      ON user_id = qa.forum_qa_author_id 
      LEFT JOIN (SELECT * 
         FROM votes) AS v 
      ON qa.forum_qa_id = v.forum_qa_id_fk 
      LEFT JOIN (SELECT forum_cm_id, 
          forum_cm_author_id, 
          forum_qa_id_fk, 
          forum_cm_text, 
          forum_cm_timestamp, 
          forum_cm_flag, 
          first_name AS forum_cm_first_name, 
          last_name AS forum_cm_last_name, 
          facebook_id AS forum_cm_fb_id, 
          picture  AS forum_cm_picture, 
          moderator AS forum_cm_moderator 
         FROM forum_cm 
          JOIN user_profiles 
           ON user_id = forum_cm_author_id) AS c 
      ON qa.forum_qa_id = c.forum_qa_id_fk 
      LEFT JOIN (SELECT forum_qa_parent_id AS pid, 
          COUNT(*)   AS ans_count 
         FROM forum_qa 
         WHERE forum_qa_parent_id IS NOT NULL 
         GROUP BY forum_qa_parent_id) AS n 
      ON qa.forum_qa_id = n.pid 
    WHERE qa.forum_qa_id LIKE "%" 
      AND qa.forum_qa_parent_id IS NULL 
    ORDER BY qa.forum_qa_timestamp DESC 

(VA BIEN) LA SOLUTION

Après avoir lu ici

http://www.mysqlperformanceblog.com/2006/09/01/order-by-limit-performance-optimization/

j'ai décidé d'indexer le champ que je voulais ORDER BY (forum_qa_type) - une fois que j'ai fait cela, la requête retourne le nombre correct de questions.

Merci à Michael d'avoir aidé. Mettra à jour ici si cela cesse de fonctionner.

Answer 1

En supposant que forum_qa est la table contenant les questions, vous pouvez placer son SELECT * dans une sous-requête avec sa propre limite. Ceci est bien sûr non testé, mais devrait fonctionner en principe.

SELECT  qa.*, 
      user_profiles.*, 
      c.*, 
      n.pid, 
      v.*, 
      Ifnull(n.ans_count, 0) AS ans_count 
    FROM (SELECT * FROM forum_qa LIMIT 0, 3) qa 
      JOIN user_profiles 
      ON user_profiles.user_id = qa.forum_qa_author_id 
      LEFT JOIN (SELECT *..... 
      -- etc... 

références supplémentaires au nom de la table forum_qa devront être changé à son nouvel alias qa.