mysql> select * from tba;
+------+------+
| id   | name |
+------+------+
|    18 | na     |
+------+------+
1 row in set (0.00 sec)

mysql> select * from tbb;
+------+------+
| id   | a_id|
+------+------+
|   10 | 18   |
|   13 | 18   |
+------+------+
2 rows in set (0.00 sec)

mysql> select * from tbc;
+------+------+
| id   |  a_id
+------+------+
|  101 | 18   |
|  120 | 18   |
|  130 | 18   |
+------+------+
3 rows in set (0.00 sec)
怎么不让他笛卡尔积分
想要这样的数据
+------+------+------+
| c    | b    | a_id |
+------+------+------+
| 101  | NULL | 18   |
| 120  | NULL | 18   |
| 130  | NULL | 18   |
| NULL | 10   | 18   |
| NULL | 13   | 18   |
+------+------+------+
先谢谢各位