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 | 12345678901234567890123456789012 | NULL | | 2 | name2 | 12345678901234567890123456789012 | 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 | 12345678901234567890123456789012 | 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 | 12345678901234567890123456789012 | 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
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