如何获取最新的2行（PostgreSQL）

Question

我需要您的帮助来修改查询以完成要求。

根据GATHER_TIME，我想显示名称的值。 该查询每1分钟运行一次，并且应始终获得2行。 如果当时没有数据，则应获取Name的过去（前1分钟）数据。

在这里，具体要求是我不应该在Timestamp值（GATHER_TIME）中考虑秒值。

我试图进行如下查询。 它仅获取最新数据。

你能帮我吗？

询问

select NAME,sum(VALUE)
FROM TestTable
WHERE substring(to_char(gather_time,'YYYY-MM-DD HH24:MI:SS'),1,16) IN ( SELECT MAX(substring(to_char(gather_time,'YYYY-MM-DD HH24:MI:SS'),1,16)) FROM TestTable )
GROUP BY NAME

表数据

NAME    COL1   COL2    GATHER_TIME         VALUE
------------------------------------------------
first   prince PQ1     2015-12-29 13:10:33 11
first   prince PQ2     2015-12-29 13:10:33 14
first   prince PQ3     2015-12-29 13:10:33 18
first   prince PQ4     2015-12-29 13:10:33 19
second  prince TT1     2015-12-29 13:10:59 20
second  prince TT2     2015-12-29 13:10:59 29
second  prince TT3     2015-12-29 13:10:59 43
first   prince PQ1     2015-12-29 13:11:37 71
first   prince PQ2     2015-12-29 13:11:37 74
first   prince PQ3     2015-12-29 13:11:37 78
first   prince PQ4     2015-12-29 13:11:37 79

所需结果

@ Query time: 2015-12-29 13:10:59
first       62
second      92

@ Query time: 2015-12-29 13:11:59
first       302
second      92

添加：

抱歉。 我应该放更详细的测试数据。 更新了它。

如果GROUP BY中使用了两列：:（需要验证）

SELECT DISTINCT ON (name,col1,gather_time) name,col1,SUM(value) 
FROM test_table
GROUP BY name,col1,gather_time
ORDER BY gather_time DESC
LIMIT 2;

Answer 1

-- DDL
CREATE TABLE test_table(
  name TEXT,
  col2 TEXT,
  gather_time TIMESTAMP,
  value INTEGER
);

--sample data
INSERT INTO test_table  VALUES
  ('first','PQ1','2015-12-29 13:10:33'::TIMESTAMP, 11),
  ('first','PQ2','2015-12-29 13:10:33'::TIMESTAMP, 14),
  ('first','PQ3','2015-12-29 13:10:33'::TIMESTAMP, 18),
  ('first','PQ4','2015-12-29 13:10:33'::TIMESTAMP, 19),
  ('second','TT1','2015-12-29 13:10:59'::TIMESTAMP, 20),
  ('second','TT2','2015-12-29 13:10:59'::TIMESTAMP, 29),
  ('second','TT3','2015-12-29 13:10:59'::TIMESTAMP, 43),
  ('first','PQ1','2015-12-29 13:11:37'::TIMESTAMP, 71),
  ('first','PQ2','2015-12-29 13:11:37'::TIMESTAMP, 74),
  ('first','PQ3','2015-12-29 13:11:37'::TIMESTAMP, 78),
  ('first','PQ4','2015-12-29 13:11:37'::TIMESTAMP, 79);

--query to run every minute
SELECT name,SUM(value) FROM test_table
  WHERE gather_time <= now()
  GROUP BY name,gather_time
  ORDER BY gather_time DESC
  LIMIT 2;

或使用功能：

CREATE OR REPLACE FUNCTION get_2latest_agg_rows(qtime timestamp) 
  RETURNS TABLE (
    order_name TEXT,
    sum_value BIGINT
 ) AS $$
BEGIN
  RETURN QUERY
  SELECT name as order_name,SUM(value) as agg_result FROM test_table
  WHERE gather_time <= qtime::TIMESTAMP
  GROUP BY name,gather_time
  ORDER BY gather_time DESC
  LIMIT 2;
END;
$$ LANGUAGE plpgsql;

--sample run
SELECT * FROM get_2latest_agg_rows('2015-12-29 13:11:59'::TIMESTAMP);

Answer 2

如果您将按collect_time排序并使用限制2来获取最新的2行怎么办？ 符合要求吗？

select NAME,sum(VALUE) FROM TestTable ORDER BY gather_time DESC LIMIT 2;