SQL左联接：选择最后一条记录

Question

我有一个项目表和一个意见表。 一个项目可以有很多评论。 我想获取所有发布的评论> 30天或没有评论的项目的列表。 做到这一点的最佳方法是什么？

我尝试了许多失败的尝试； 这是我最新的尝试。

SELECT p.id, 
       p.officialStatus, 
       c.posted 
  FROM projects p 
       LEFT JOIN 
       (
        SELECT max(posted) as posted, 
                   projectid 
          FROM comments 
             WHERE DATEDIFF(day, posted, GETDATE()) > 30 
                   OR comment IS NULL
               group by projectid
        ) c ON p.id = c.projectid 
 WHERE (p.officialStatus NOT IN ('Blue', 'Canceled'))

请在您的答案中使用这些表/列名称：

• 项目：id，officialStatus
• 评论：id，projectID，postedOn

Answer 1

SELECT projects.id FROM projects
  LEFT JOIN
    (SELECT comments.projectID 
       FROM comments
      GROUP BY comments.projectID
      HAVING DATEDIFF(Now(), MAX(comments.postedOn)) < 30) AS C
  ON projects.id = C.projectID
  WHERE C.projectID IS NULL;

http://sqlfiddle.com/#!2/ec919/14

Answer 2

 SELECT PROJ.id,
        PROJ.officialStatus
   FROM Projects PROJ
        LEFT JOIN
        (
            SELECT projectid, MAX(posted) AS max_posted
              FROM Comments 
                   GROUP BY projectid
        )  COMMENTS ON PROJ.id = COMMENTS.projectid
  WHERE PROJ.officilstatus NOT IN ('Blue', 'Cancelled)
        AND COMMENTS.max_posted IS NULL
         OR COMMENTS.max_posted >= DATEADD(day, -30, Now())

我认为您的主要问题是外部联接，这正是您所不需要的...在此调整中，没有注释的项目的max_posted日期为NULL。

Answer 3

select p.*, max(c.postedOn) as postedDate from Projects p
left join Comments c on p.id = c.projectId
group by p.id
having COALESCE( postedDate, DATE_SUB(CURRENT_DATE, INTERVAL 31 DAY ) ) < 
    DATE_SUB(CURRENT_DATE, INTERVAL 30 DAY );

http://sqlfiddle.com/#!2/ec919/17-以及执行计划（感谢@Barbara在sqlfiddle上准备此测试）