Mysql query to retrieve the latest updates based on DATETIME for each parameter
I think you may be looking for something like this:

SELECT s.id,s.station_code,s.parameter_code,s.value,s.date_time
  FROM station_data s
 WHERE s.date_time = (SELECT MAX(date_time) FROM station_data WHERE station_code = s.station_code AND paramter_code = s.parameter_code)


Add whichever parameters are important for uniqueness in your sub-query.

I would also recommend for this to run optimally, you create an index that indexes the three columns in that index, namely date_time,station_code and parameter_code)