Question

我的查询：

   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

我的问题是第三行的“LIMIT 1”。这将所有评论仅限于最新评论，因此只有最新评论的问题才会被报告为评论。

如果我从那里删除了“LIMIT 1”部分，我会在一个问题的每个评论中找到一行，这不是我想要的。我想要的只是每个问题的最新评论。

无论如何，我不确定我的IFNULL部分是否正常工作，因为那不是我在调试的地方。

那么我如何实现我的目标呢？

Answer 1

尝试：

   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 2

   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 3

修改

由于MySql毕竟没有CTE，试试这个：

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

LEFT JOIN上的MySQL LIMIT

3 个答案: