selecting a column based on a minimum value of ano

问题:

I have the following table.

test_type |  brand  | model  | band | firmware_version | avg_throughput
-----------+---------+--------+------+-----------------+----------------
 1client   | Linksys | N600   | 5ghz | 1               |          66.94
 1client   | Linksys | N600   | 5ghz | 2               |          94.98
 1client   | Linksys | N600   | 5ghz | 4               |         132.40
 1client   | Linksys | EA6500 | 5ghz | 1               |         216.46
 1client   | Linksys | EA6500 | 5ghz | 2               |         176.79
 1client   | Linksys | EA6500 | 5ghz | 4               |         191.44

I'd like to select the avg_throughput of each model that has the lowest firmware_version.

When I do SELECT test_type, model, min(firmware_version) FORM temp_table GROUP BY test_type, model I get what I want but once I add the avg_throughput column it requires me to also add it to the GROUP BY clause which makes it return all the rows when all I need is only the avg_throughput for the lowest firmware_version for each model type.

回答1:

In standard SQL this can be done using a window function

select test_type, model, firmware_version, avg_throughput
from (
  select test_type, model, firmware_version, avg_throughput, 
         min(firmware_version) over (partition by test_type, model) as min_firmware
  from temp_table
) t
where firmware_version = min_firmware;

Postgres however has the distinct on operator which is usually faster than the corresponding solution with a window function:

select distinct on (test_type, model) 
       test_type, model, firmware_version, avg_throughput
from temp_table
order by test_type, model, firmware_version;

SQLFiddle example: http://sqlfiddle.com/#!15/563bd/1

回答2:

This should be what you're looking for if I'm reading your post correctly, and I think it's a pretty easily readable way of doing it. :-)

WITH min_firmware_version (model, firmware_version)
AS
(
    SELECT
        model,
        MIN(firmware_version)
    FROM temp_table
    GROUP BY
        model
)
SELECT
    temp_table.model,
    temp_table.firmware_version,
    temp_table.avg_throughput
FROM temp_table
INNER JOIN min_firmware_version
    ON temp_table.model = min_firmware_version.model
    AND temp_table.firmware_version = min_firmware_version.firmware_version

回答3:

I think you need that SQL statement:

SELECT t.test_type, t.model, t.firmware_version, t.avg_throughput 
FROM temp_table t 
WHERE t.firmware_version = (SELECT min(firmware_version) FROM temp_table)