How to add corresponding date to grouped max/min-value in postgres?

Question

I have a climate timeseries table with different measured parameters for many stations over some years (daily values). I am using postgres 9.4 with pgadmin.

The table looks this way:

Table name kl

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

my select Code:

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

gives out the max-temperature value for every station: table

Now the question: how to add for every T_max value the corresponding date in another column (the date on which that max value was measured)?

thanks for your help

Answer 1

You use row_number() to get the whole row

PARTITION BY reset the row counter for each station, so you wont need 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 

And just change * for the field names you need

Answer 2

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

Use the date column (bad name) to untie:

order by stat_id, temperatur desc, date

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

If you want both the min and max temperatures in the same query:

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