现在我们先来把test表中的一条记录的birth字段设置为空。

mysql> update test set t_birth=null where t_id=1;
Query OK, 1 row affected (0.02 sec)
Rows matched: 1 Changed: 1 Warnings: 0

OK，执行成功！
设置一个字段值为空时的语法为：set <字段名>=NULL
说明一下，这里没有大小写的区分，可以是null，也可以是NULL。

下面看看结果：

mysql> select * from test;
+------+--------+----------------------------------+------------+
| t_id | t_name | t_password | t_birth |
+------+--------+----------------------------------+------------+
| 1 | name1 | 12345670123456701234567012 | NULL |
| 2 | name2 | 12345670123456701234567012 | 2013-01-01 |
+------+--------+----------------------------------+------------+
2 rows in set (0.00 sec)

接下来分别查询一下字段t_birth值为空或不为空的记录：

mysql> select * from test where t_birth is null;
+------+--------+----------------------------------+---------+
| t_id | t_name | t_password | t_birth |
+------+--------+----------------------------------+---------+
| 1 | name1 | 12345670123456701234567012 | NULL |
+------+--------+----------------------------------+---------+
1 row in set (0.00 sec)

mysql> select * from test where t_birth is not null;
+------+--------+----------------------------------+------------+
| t_id | t_name | t_password | t_birth |
+------+--------+----------------------------------+------------+
| 2 | name2 | 12345670123456701234567012 | 2013-01-01 |
+------+--------+----------------------------------+------------+
1 row in set (0.00 sec)

说明：
1、查询字段值为空的语法：where <字段名> is null
2、查询字段值不为空的语法：where <字段名> is not null