16

我意识到数据库查询返回了意外结果，这与我对“DISTINCT ON”和“GROUP BY”的不当使用有关

我希望有人可以让我明白这一点。实际的查询非常复杂，所以我将其简化：

我有一个由 object_id 和时间戳组成的表/内部查询：

CREATE TABLE test_select ( object_id INT , event_timestamp timestamp );
COPY test_select (object_id , event_timestamp) FROM stdin (DELIMITER '|');
1           | 2013-01-27 21:01:20
1           | 2012-06-28 14:36:26
1           | 2013-02-21 04:16:48
2           | 2012-06-27 19:53:05
2           | 2013-02-03 17:35:58
3           | 2012-06-14 20:17:00
3           | 2013-02-15 19:03:34
4           | 2012-06-13 13:59:47
4           | 2013-02-23 06:31:16
5           | 2012-07-03 01:45:56
5           | 2012-06-11 21:33:26
\.

我正在尝试选择一个不同的 ID ，由反向 chron 上的时间戳排序/去重

所以结果应该是 [ 4, 1, 3, 2, 5 ]

我认为这可以满足我的需要（似乎）：

SELECT object_id  
FROM test_select 
GROUP BY object_id 
ORDER BY max(event_timestamp) DESC
;

出于测试/审计目的，我有时想包含时间戳字段。我似乎无法弄清楚如何在该查询中包含另一个字段。

谁能指出我上面的 sql 中的明显问题，或者关于如何包含审计信息的建议？

4

2 回答 2

22

为了能够选择所有列而不仅仅是object_idand MAX(event_timestamp)，您可以使用DISTINCT ON

SELECT DISTINCT ON (object_id) 
    object_id, event_timestamp    ---, more columns
FROM test_select 
ORDER BY object_id, event_timestamp DESC ;

如果您想要按event_timestamp DESC而不是按排序的结果object_id，则需要将其包含在派生表或 CTE 中：

SELECT *
FROM 
  ( SELECT DISTINCT ON (object_id) 
        object_id, event_timestamp    ---, more columns
    FROM test_select 
    ORDER BY object_id, event_timestamp DESC 
  ) AS t
ORDER BY event_timestamp DESC ;

或者，您可以使用窗口函数，例如ROW_NUMBER()：

WITH cte AS
  ( SELECT ROW_NUMBER() OVER (PARTITION BY object_id 
                              ORDER BY event_timestamp DESC) 
             AS rn, 
           object_id, event_timestamp    ---, more columns
    FROM test_select 
  )
SELECT object_id, event_timestamp    ---, more columns
FROM cte
WHERE rn = 1
ORDER BY event_timestamp DESC ;

或聚合MAX()：OVER

WITH cte AS
  ( SELECT MAX(event_timestamp) OVER (PARTITION BY object_id) 
             AS max_event_timestamp, 
           object_id, event_timestamp    ---, more columns
    FROM test_select 
  )
SELECT object_id, event_timestamp    ---, more columns
FROM cte
WHERE event_timestamp = max_event_timestamp
ORDER BY event_timestamp DESC ;
于 2013-09-22T01:46:50.967 回答
5

这可能不是处理此问题的最佳方法，但您可以尝试使用窗口函数：

SELECT DISTINCT object_id, MAX(event_timestamp)
OVER (PARTITION BY object_id)  
FROM test_select ORDER BY max DESC;

另一方面，它也可以：

SELECT object_id, MAX(event_timestamp) as max_event_timestamp
FROM test_select 
GROUP BY object_id 
ORDER BY max_event_timestamp DESC;
于 2013-09-22T01:05:50.700 回答