id | group_id | str     | position_number
 1 |        1 | first   | 1
 2 |        1 | string  | 2
 3 |        2 | And     | 1
 4 |        2 | another | 2
 5 |        2 | string  | 3

group_id | str
       1 | firststring
       2 | Andanotherstring

SELECT
group_id
, GROUP_CONCAT(str SEPARATOR '' ORDER BY position_number)
FROM
your_table
GROUP BY group_id