为什么显示结果不存在
mysql> SELECT *FROM test;
+-----------+----------+
| firstname | lastname |
+-----------+----------+
| A | B |
| C | D |
| tom% | 123 |
| NULL | 11 |
+-----------+----------+
4 rows in set (0.00 sec)
mysql> SELECT *FROM test WHERE firstname IS NULL;
Empty set (0.00 sec)
