mysql中两张表使用left join on 求差集

mysql> select * from allStudents;
+----+-------+
| id | name  |
+----+-------+
|  1 | ????  |
|  2 | ????  |
|  3 | ???·   
|  4 | four  |
+----+-------+
4 rows in set (0.00 sec)

mysql> select * from currentStudents;
+----+--------+
| id | name   |
+----+--------+
|  1 | luowen |
|  3 | 毛毛想 |
+----+--------+
mysql> select * from allStudents;
+