weather_observation_station_20.sql

SET 
  @ROWINDEX := -1;
SELECT 
  ROUND(
    AVG(T.LAT_N), 
    4
  ) 
FROM 
  (
    SELECT 
      @ROWINDEX := @ROWINDEX + 1 AS ROWINDEX, 
      S.LAT_N 
    FROM 
      STATION AS S 
    ORDER BY 
      S.LAT_N
  ) AS T 
WHERE 
  T.ROWINDEX IN (
    FLOOR(@ROWINDEX / 2), 
    CEIL(@ROWINDEX / 2)
  );