PostgreSQL查詢中的挑戰（分組和有問題）

Question

我試圖創建一個查詢，但我遇到了一些麻煩。我有兩個表：

• 用戶（ID，姓名，電子郵件）
• 評論（ID，UID，評論，日期，時間）

我想列出所有用戶和他們的評論，這可以通過內部連接很容易地完成。但是，我收到每個用戶的各種評論，因爲我加入了結果。我只想要他們的最新評論。有任何想法嗎？ :)

Answer 1

對於PostgreSQL 8.4+：

SELECT x.* 
    FROM (SELECT u.*, c.*, 
       ROW_NUMBER() OVER (PARTITION BY u.id 
             ORDER BY c.date DESC, c.time DESC) AS rnk 
      FROM USERS u 
      JOIN COMMENTS c ON c.uid = u.id) x 
WHERE x.rnk = 1 

Answer 2

這可能會實現：

編輯：

我查詢更新到這一點：

SELECT u.id, u.name, u.email, t.id, t.uid, t.comment, t.date, t.time 
FROM users u 
LEFT OUTER JOIN 
(
    select c.id, m.uid, c.comment, m.cdate, c.time 
    from comments c 
    right outer join 
     (
      select uid, max(date) as cdate 
      from comments 
      group by uid 
     ) as m 
    ON c.cdate = m.cdate 
) t 
ON u.id = t.uid 

Answer 3

假設評論的ID是autoincrement，找到最高紀錄每個用戶（最新評論）

SELECT u.id, u.name, u.email, c.id, c.uid, c.comment, c.date, c.time 
FROM users u 
JOIN comments c ON u.id = c.uid 
JOIN 
    (
     select uid, max(id) id 
     from comments 
     group by uid 
    ) as c2 ON c.id = c2.id AND c.uid = c2.uid 

Answer 4

OMG小馬肯定有最好的答案，但這裏是另一種方式來做到這一點，沒有任何擴展數據庫功能：

select 

u.name, 
c.comment, 
c.comment_date_time 

from users as u 
left join comments as c 
on c.uid = u.id 
and 
c.comment_date_time - 
(
    select max(c2.comment_date_time) 
    from comments as c2 
    where c2.uid = u.id 
) = 0 

我有合併的日期和時間列到comment_date_time在這個例子中。

Answer 5

這應該這樣做：

select distinct on(u.name, u.id) * 
from comments c, users u 
where u.id=c.uid 
order by u.name, u.id, c.date desc