如何在postgres中將相應的日期添加到分組的最大值/最小值？

Question

我有一個氣候時間序列表，其中某些年份的許多台站的測量參數不同（每日值）。 我正在使用pgadmin的postgres 9.4。

該表如下所示：

表名稱kl

station_id [int], 
date [date], 
temperature [numeric] ...

我選擇的代碼：

select 
  stat_id,
  max(temperatur) as "T_max"
from kl 
group by stat_id
order by stat_id

給出每個站點的最高溫度值： 表

現在的問題是：如何為每個T_max值添加另一列中的對應日期（測量最大值的日期）？

謝謝你的幫助

Answer 1

您使用row_number（）獲取整行

PARTITION BY重置每個工作站的行計數器，因此您不需要group by 。

WITH cte as ( 
     SELECT *,
            ROW_NUMBER() OVER (PARTITION BY station_id 
                               ORDER BY temperature DESC) AS rn
     FROM kl 
) 
SELECT *
FROM cte
WHERE rn = 1 

只需將*更改為所需的字段名稱

Answer 2

select distinct on (stat_id)
    stat_id, temperatur, date
from kl 
order by stat_id, temperatur desc

使用date列（不正確的名稱）可以取消綁定：

order by stat_id, temperatur desc, date

http://www.postgresql.org/docs/current/static/sql-select.html#SQL-DISTINCT

如果要在同一查詢中同時使用最低和最高溫度：

with kl (stat_id, temperatur, date) as (values
    (1, 17.1, '2015-01-01'::date), (1, 17.2, '2015-01-02')
)
select stat_id,
    t_max[1]::numeric as t_max,
    (date 'epoch' + t_max[2] * interval '1 second')::date as d_max,
    t_min[1]::numeric as t_min,
    (date 'epoch' + t_min[2] * interval '1 second')::date as d_min
from (
    select
        stat_id,
        max(array[temperatur, extract(epoch from date)::numeric]) as t_max,
        min(array[temperatur, extract(epoch from date)::numeric]) as t_min
    from kl
    group by 1
) s
;
 stat_id | t_max |   d_max    | t_min |   d_min    
---------+-------+------------+-------+------------
       1 |  17.2 | 2015-01-02 |  17.1 | 2015-01-01