MySQL group by after union＃1064 error

我有2个表格，我必须从日期得到最后10个活动项目ID。查询是：MySQL group by after union＃1064 error

(
SELECT project_posts.projectid as projectId, `project_posts`.`date` as rowDate 
FROM `project_posts` ORDER BY `project_posts`.id DESC 
) 
UNION ALL 
(SELECT `project_comments`.projectid as projectId, project_comments.`date` as rowDate FROM `project_comments` ORDER BY `project_comments`.id DESC) GROUP BY projectId ORDER BY rowDate DESC LIMIT 10

而我想通过rowDate DESC（最后项目第一）对它排序。问题是，当我被MySQL添加组说：

 
#1064 - You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use 
near 'GROUP BY projectId ORDER BY rowDate DESC LIMIT 6' at line 9

我看着在互联网上与工会，工会都和group by的问题，但我没有找到任何解决方案。在此先感谢。

来源

2012-03-01 some12die4

我认为你需要，才能在IT团队从联合查询创建派生表 - 我还没有机会后，试试这个，但它应该做的你是什么

SELECT projectId, MAX(rowDate) AS rowDate 
FROM (
    (
     SELECT project_posts.projectid as projectId, `project_posts`.`date` as rowDate 
     FROM `project_posts` 
    ) UNION ALL (
     SELECT `project_comments`.projectid as projectId, project_comments.`date` as rowDate 
     FROM `project_comments` 
    ) 
) AS tmp 
GROUP BY projectId 
ORDER BY rowDate DESC 
LIMIT 10

SELECT p.id, GREATEST(IFNULL(MAX(pc.date), 0), IFNULL(MAX(pp.date), 0)) AS max_date 
FROM projects p 
LEFT JOIN project_comments pc 
    ON p.id = pc.projectid 
LEFT JOIN project_posts pp 
    ON p.id = pp.projectid 
GROUP BY p.id 
ORDER BY max_date DESC 
LIMIT 10

UPDATE的UNION基于查询应该比这个第二查询显著快。我已经使用一些虚拟数据（10万个项目，20万个帖子和700万条评论）做了一些粗略的测试，并且多联接查询比UNION查询长3到5倍。

来源

2012-03-01 20:49:04 nnichols

非常感谢您的快速回复。现在查询运行，但排序不正确，我没有拿到最后的项目ID（当我为日期DESC排序的两个塔布运行单独的select时，它们是不同的）。我尝试用ORDER BY'project_comments'.'date' DESC和'project_posts'.'date' DESC来更改每个select（在UNION中）的ORDER BY子句，但它仍然存在。任何建议？提前致谢。 – some12die4 2012-03-01 21:04:00

我刚刚阅读您的原始文章。到底什么是最终目标和什么是表结构？我不认为用UNION进行查询是最好的方法。 – nnichols 2012-03-01 21:26:50

对不起，误会。我想根据他们的活动（帖子和评论）获取最近10个活动项目。我没有找到与工会不同的方式。 – some12die4 2012-03-01 21:40:10

MySQL group by after union＃1064 error

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