MySQL LIMIT LEFT JOIN

Question

Ma requête:

SELECT issues.*, 
      comments.author AS commentauthor, 
      comments.when_posted AS commentposted 
    FROM issues 
LEFT JOIN (SELECT * 
      FROM comments 
     ORDER BY when_posted DESC 
      LIMIT 1) AS comments ON issues.id=comments.issue 
ORDER BY IFNULL(commentposted, issues.when_opened) DESC 

Mon problème est le "LIMIT 1" sur la troisième ligne. Cela limite tous les commentaires au plus récent, ainsi seuls les problèmes avec le commentaire le plus récent seront rapportés comme ayant un commentaire.

Si je supprimais la partie "LIMIT 1" à partir de là, j'obtiendrais une ligne pour chaque commentaire d'un problème, et ce n'est pas ce que je veux. Ce que je veux, c'est seulement le commentaire le plus récent pour chaque numéro.

Dans tous les cas, je ne suis pas sûr que ma partie IFNULL fonctionne même parce que ce n'est pas ce que je suis en train de faire pour le débogage.

Alors, comment pourrais-je réaliser ce que je voulais?

Answer 1

SELECT issues.*, 
      comments.author AS commentauthor, 
      comments.when_posted AS commentposted 
    FROM issues 
LEFT JOIN (SELECT c1.issue, c1.author, c1.when_posted 
       FROM comments c1 
      JOIN 
      (SELECT c2.issue, max(c2.when_posted) AS max_when_posted   
       FROM comments c2 
      GROUP BY issue) c3 
      on c1.issue = c3.issue and c1.when_posted = c3.max_when_posted 
     ) AS comments ON issues.id=comments.issue 
ORDER BY COALESCE(commentposted, issues.when_opened) DESC 

Answer 2

Essayez:

SELECT i.*, 
      c.author AS commentauthor, 
      c.when_posted AS commentposted 
    FROM ISSUES i 
LEFT JOIN COMMENTS c ON c.issue = i.id 
    JOIN (SELECT c.issue, 
        MAX(c.when_posted) 'max_when_posted' 
      FROM COMMENTS c 
     GROUP BY c.issue) mwp ON mwp.issue = c.issue 
           AND mwp.max_when_posted = c.when_posted 
ORDER BY COALESCE(c.when_posted, i.when_opened) DESC 

Answer 3

Modifier

Depuis MySql n'a pas CTE après tout, essayez ceci:

SELECT i.* 
    c.author AS CommentAuthor, 
    c.when_posted AS CommentPosted 
FROM Issues i 
LEFT JOIN 
    (SELECT issue, MAX(when_posted) AS LastPostDate 
    FROM comments GROUP BY issue) ic ON ic.issue = i.id 
LEFT JOIN Comment c ON c.issue = i.id AND c.when_posted = ic.LastPostDate 
ORDER BY COALESCE(ic.LastPostDate, issues.when_opened) DESC