MySQL data table insert content null and non-empty (vi)

NULL and NOT NULL

mysql> CREATE TABLE tb2(
-Usename VARCHAR (a) not NULL,
-Age TINYINT UNSIGNED null (can be empty)
);
Query OK, 0 rows affected (0.06 sec)

Mysql> SHOW COLUMNS from TB2;
+---------+---------------------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+---------+---------------------+------+-----+---------+-------+
| Usename | varchar (20) | NO | | NULL | |
| Age | tinyint (3) unsigned | YES | | NULL | |
+---------+---------------------+------+-----+---------+-------+
2 rows in Set (0.01 sec)

Mysql>
mysql> INSERT tb2 VALUES (' Tom ', NULL);
Query OK, 1 row affected (0.03 sec)

Mysql> SELECT * from TB2;
+---------+------+
| Usename | Age |
+---------+------+
| Tom | NULL |
+---------+------+
1 row in Set (0.00 sec)

mysql> INSERT tb2 VALUES (NULL, n);
ERROR 1048 (23000): Column ' usename ' cannot be null
Mysql>

MySQL data table insert content null and non-empty (vi)