+----+-------------+----------+--------+
| id | external_id | group_id | mypath |
+----+-------------+----------+--------+
|  1 |        1003 |        1 | a      |
|  2 |        1004 |        2 | b      |
|  3 |        1005 |        2 | c      |
+----+-------------+----------+--------+

select * from temp GROUP BY group_id

+----+-------------+----------+--------+
| id | external_id | group_id | mypath |
+----+-------------+----------+--------+
|  1 |        1003 |        1 | a      |
|  2 |        1004 |        2 | b      |
+----+-------------+----------+--------+

+----+-------------+----------+--------+
| id | external_id | group_id | mypath |
+----+-------------+----------+--------+
|  1 |        1003 |        1 | a      |
|  3 |        1005 |        2 | c      |
+----+-------------+----------+--------+

SELECT
   t1.group_id, some_table.id, some_table.mypath
FROM
   (
      SELECT group_id, MAX(external_id) AS external_id
      FROM some_table
      GROUP BY group_id
   ) as t1
INNER JOIN
   sometable ON t1.external_id = sometable.external_id
WHERE ...