Question

我有一张名为＆＃34; AGGREGATEVALUE＆＃34;在数据库模式＆＃34; CLOUD＆＃34;。我正在使用PostgreSQL：

CREATE TABLE CLOUD.AGGREGATEVALUE(
    id SERIAL,
    value CLOUD.FLOATVALUE NOT NULL,
    idBoard VARCHAR(32),
    idParameterRegolator VARCHAR(32),
    timestamp TIMESTAMP NOT NULL DEFAULT CURRENT_TIMESTAMP,
    PRIMARY KEY (id)
    FOREIGN KEY(idBoard) REFERENCES CLOUD.BOARD(id),
    FOREIGN KEY(idParameterRegolator) REFERENCES CLOUD.PARAMETERREGOLATOR(id)
);

以下是此表格实例的示例（使用简单选择获取所有关于idboard='board1'的内容）

id | value | idboard | idparameterregolator |         timestamp          
----+-------+---------+----------------------+----------------------------
  1 |  1.00 | board1  | param1               |       | 2014-08-24 10:29:33.677425
  2 |  2.00 | board1  | param1               |       | 2014-08-24 10:29:39.507868
  3 |  4.00 | board1  | param1               |       | 2014-08-24 10:29:46.286815
  4 |  5.00 | board1  | param1               |       | 2014-08-24 10:29:53.513363
  5 |  1.00 | board1  | param2               |       | 2014-08-24 10:29:58.252255
  6 |  2.00 | board1  | param2               |       | 2014-08-24 10:30:02.265151
  7 |  3.00 | board1  | param2               |       | 2014-08-24 10:30:05.480344
  8 |  4.00 | board1  | param2               |       | 2014-08-24 10:30:08.281917
  9 |  5.00 | board1  | param2               |       | 2014-08-24 10:30:10.845078
 10 |  1.00 | board1  | param3               |       | 2014-08-24 10:30:15.708794
 11 |  2.00 | board1  | param3               |       | 2014-08-24 10:30:18.82404
 12 |  3.00 | board1  | param3               |       | 2014-08-24 10:30:22.082542
 13 |  4.00 | board1  | param3               |       | 2014-08-24 10:30:24.662375
 14 |  5.00 | board1  | param3               |       | 2014-08-24 10:30:27.994689

假设从上表开始，我需要的是获得另一张表：

value | idparameterregolator | timestamp
----------------------------------------------------------
5.00  | param1               | 2014-08-24 10:29:53.513363
5.00  | param2               | 2014-08-24 10:30:10.845078
5.00  | param3               | 2014-08-24 10:30:27.994689

也就是说，我需要为每个参数获取具有最后一个时间戳值的相应值。
我尝试使用ORDER BY子句但没有任何结果。

Answer 1

具有distinct on 2值的

order by可以解决问题。

SELECT distinct on(value) * 
FROM   CLOUD.AGGREGATEVALUE 
ORDER  BY value, TIMESTAMP DESC;

Answer 2

标准方式，避免聚合：

SELECT * FROM AGGREGATEVALUE av
WHERE NOT EXISTS (
    SELECT * FROM AGGREGATEVALUE nx
    WHERE nx.idparameterregolator =av.idparameterregolator
    AND nx.idboard = av.idboard
    AND nx.ztimestamp > av.ztimestamp
    );

注意：我将timestamp替换为ztimestamp，因为它是一个保留字。

为每个字段选择具有最后一个时间戳值的字段

2 个答案: