sql - PostgreSQL - "DISTINCT ON"和 "GROUP BY"语法

Question

我意识到数据库查询返回了意外的结果，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，按反向时间戳进行排序/去重

所以结果应该是 [ 4, 1, 3, 2, 5 ]

我认为这满足了我的需要(似乎是):

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

出于测试/审计目的，有时我想包含时间戳字段。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 中明显的问题，或者关于如何包含审计信息的建议？

Answer 1

要能够选择所有列而不仅仅是 object_id 和 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 ;

如果您希望结果按 event_timestamp DESC 而不是按 object_id 排序，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() 与 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 ;