在已经很复杂的SQL语句之后，每个用户最近有两条记录（对我而言）

Question

我是SQL的新手。 我有两个表，我连接内连接，排序和过滤。 然后我只需要显示按“用户名”分组的两个最新条目。 我不知道该怎么做。 我已经查看了一些已发布的解决方案，但我不知道如何将这些解决方案应用到我的情况中。

这就是我现在的位置：

SELECT users.username, logs.event, logs.insertdate
FROM users 
INNER JOIN logs ON users.identifier = logs.useridentifier
WHERE (((logs.event)="Taken" Or (logs.event)="Return"))
ORDER BY users.username, logs.insertdate DESC;

这是我得到的输出的一个例子：

|username | event | insertdate | 
================================
 alan     | Return| 10/15/2014 3:17 PM
 alan     | Taken | 10/15/2014 6:49 AM
 alan     | Return| 10/14/2014 3:16 PM
 alan     | Taken | 10/14/2014 6:50 AM
 alan     | Return| 10/13/2014 3:15 PM
 bill     | Return| 10/15/2014 3:19 PM
 bill     | Taken | 10/15/2014 6:53 AM
 bill     | Return| 10/14/2014 3:26 PM
 bill     | Taken | 10/14/2014 7:00 AM
 bill     | Return| 10/13/2014 3:19 PM

等等。 我希望做的只是获得每个用户的前两个条目。 所以

|username | event | insertdate | 
================================
 alan     | Return| 10/15/2014 3:17 PM
 alan     | Taken | 10/15/2014 6:49 AM
 bill     | Return| 10/15/2014 3:19 PM
 bill     | Taken | 10/15/2014 6:53 AM

谢谢

Answer 1

使用“解析”函数row_number（）

SELECT
      username
    , event
    , insertdate
FROM (
            SELECT
                  users.username
                , logs.event
                , logs.insertdate
                , ROW_NUMBER() OVER (PARTITION BY users.username
                                     ORDER BY logs.insertdate DESC) AS rn
            FROM users
                  INNER JOIN logs
                              ON users.identifier = logs.useridentifier
            WHERE (logs.event = "Taken"
                  OR logs.event = "Return")
      ) AS derived
WHERE rn < 3
ORDER BY
      username, insertdate DESC;

此变体可能值得尝试查看它是否有助于性能（或不）：

SELECT
      users.username
    , logrn.event
    , logrn.insertdate
FROM users
      INNER JOIN (
                  SELECT
                        logs.useridentifier
                      , logs.event
                      , logs.insertdate
                      , ROW_NUMBER() OVER (PARTITION BY logs.useridentifier
                                           ORDER BY logs.insertdate DESC) AS rn
                  FROM logs
                  WHERE (logs.event = "Taken" OR logs.event = "Return")
            ) AS logrn
                  ON users.identifier = logrn.useridentifier
                        AND rn < 3
ORDER BY
      users.username, logrn.insertdate DESC
;