MariaDB [test]> select group_concat(exam), student_id, id from mongorestore_tbl as m join study_tbl as s on ( m.children regexp concat('(^|:)', s.course_id, '($|:)')) group by s.id;
+--------------------+---------------+----+
| group_concat(exam) | student_id    | id |
+--------------------+---------------+----+
| exam3              | foo@bar.com   |  1 |
| exam3              | can@haz.com   |  2 |
| exam3              | bar@foo.com   |  3 |
| exam3              | haz@can.com   |  4 |
| exam3              | haz@can.com   |  5 |
| exam2              | haz@can.com   |  6 |
| exam3              | haz@can.co.nz |  7 |
| exam2              | haz@can.eu    |  8 |
+--------------------+---------------+----+
8 rows in set (0.00 sec)