MySQL group by after union＃1064 error

Question

我有2個表格，我必須從日期得到最後10個活動項目ID。查詢是：

(
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 

而我想通過rowDate DESC（最後項目第一）對它排序。問題是，當我被MySQL添加組說：

 
#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 

我看着在互聯網上與工會，工會都和group by的問題，但我沒有找到任何解決方案。 在此先感謝。

Answer 1

我認爲你需要，才能在IT團隊從聯合查詢創建派生表 - 我還沒有機會後，試試這個，但它應該做的你是什麼

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基於查詢應該比這個第二查詢顯著快。我已經使用一些虛擬數據（10萬個項目，20萬個帖子和700萬條評論）做了一些粗略的測試，並且多聯接查詢比UNION查詢長3到5倍。