mysql> select * from a;
+------+-------+
| a_id | a_val |
+------+-------+
|    1 |     1 |
|    2 |     3 |
|    3 |     3 |
|    4 |     4 |
|    5 |     6 |
|    6 |     7 |
|    7 |     8 |
+------+-------+
7 rows in set (0.00 sec)

mysql> select * from b;
+------+-------+-------+
| b_id | b_val | a_val |
+------+-------+-------+
|    1 |     1 |     2 |
|    2 |     4 |     2 |
|    3 |     2 |     4 |
|    4 |     7 |     4 |
|    5 |    12 |     6 |
+------+-------+-------+
5 rows in set (0.00 sec)

mysql> select a.a_id, b.b_id from a left join b on b.a_val=a.a_id left join b as b_tmp on (b_tmp.a_val=a.a_id and b_tmp.b_id > b.b_id) where b_tmp.b_id IS NOT NULL order by a.a_id asc;
+------+------+
| a_id | b_id |
+------+------+
|    2 |    1 |
|    4 |    3 |
+------+------+
2 rows in set (0.00 sec)