设置NULL空值与非NOT NULL空值

mysql> CREATE TABLE four(
    -> username VARCHAR(20) NOT NULL,             
    -> age TINYINT UNSIGNED NULL                如果这个字段为空NULL可以不用书写；
    -> );
Query OK, 0 rows affected (0.01 sec)
mysql> SHOW COLUMNS FROM FOUR;                  查看数据结构；
+----------+---------------------+------+-----+---------+-------+
| Field    | Type                | Null | Key | Default | Extra |
+----------+---------------------+------+-----+---------+-------+
| username | varchar(20)         | NO   |     | NULL    |       |
| age      | tinyint(3) unsigned | YES  |     | NULL    |       |
+----------+---------------------+------+-----+---------+-------+
2 rows in set (0.02 sec)

mysql> INSERT four values ('yu',null);           添加记录 是否为空；
Query OK, 1 row affected (0.00 sec)
mysql> SELECT * FROM four;
+----------+------+                              当前age为空；
| username | age  |
+----------+------+
| yu       | NULL |
+----------+------+
1 row in set (0.00 sec)
mysql> INSERT four VALUES (NULL,21);             这里报错 用户名禁止为空；
ERROR 1048 (23000): Column 'username' cannot be null
mysql>