Calculate distance and only get one result for every trail_part_id

What I want to achieve is to get only the closest result for every trail_part_id. However I am stuck, I tried to use GROUP BY trail_part_id, but that did not work either as I got the following error message:

SELECT list is not in GROUP BY clause and contains nonaggregated column ‘p.latpoint’ which is not functionally dependent on columns in GROUP BY clause; this is incompatible with sql_mode=only_full_group_by

I know I am missing something, please put me in the right direction.

SELECT DISTINCT(trail_part_id),
                  111.045 * DEGREES(ACOS(COS(RADIANS(latpoint))
                      * COS(RADIANS(latitude))
                      * COS(RADIANS(longpoint) - RADIANS(longitude))
                      + SIN(RADIANS(latpoint))
                      * SIN(RADIANS(latitude)))) AS distance_in_km
FROM waypoints
JOIN (SELECT ' . $latitude . '  AS latpoint,  ' . $longitude . ' AS longpoint) AS p
ORDER BY distance_in_km ASC

Answer

There was a few errors in my code.

1. As @a_horse_with_no_name explained, DISTINCT is not a function. So I removed it.
2. What I had to do was to select the lowest value of “distance_in_km” by wrapping it with MIN().

After this I did get the closest waypoint, and that was it, now I can use GROUP BY trail_part_id and get the expected result.