not null default 说明不能是NULL， 并设置默认值
default 设置默认值 ， 但值也可能是NULL
mysql> create table test (id int, name varchar(10) default 'a', addr varchar(10) not null default 'b'); Query OK, 0 rows affected (0.04 sec) mysql> desc test; +-------+-------------+------+-----+---------+-------+ | Field | Type | Null | Key | Default | Extra | +-------+-------------+------+-----+---------+-------+ | id | int(11) | YES | | NULL | | | name | varchar(10) | YES | | a | | | addr | varchar(10) | NO | | b | | +-------+-------------+------+-----+---------+-------+ 3 rows in set (0.00 sec) mysql> insert into test id values(1); ERROR 1064 (42000): You have an error in your SQL syntax; check the manual that corresponds to your MariaDB server version for the right syntax to use near 'id values(1)' at line 1 mysql> insert into test id values(1); ERROR 1064 (42000): You have an error in your SQL syntax; check the manual that corresponds to your MariaDB server version for the right syntax to use near 'id values(1)' at line 1 mysql> insert into test(id) values (1); Query OK, 1 row affected (0.00 sec) mysql> insert into test(id) values (1); Query OK, 1 row affected (0.00 sec) mysql> insert into test(id) values (1); Query OK, 1 row affected (0.00 sec) mysql> select * from test; +------+------+------+ | id | name | addr | +------+------+------+ | 1 | a | b | | 1 | a | b | | 1 | a | b | +------+------+------+ 3 rows in set (0.00 sec) mysql> insert into test(id,name) values (1,null); Query OK, 1 row affected (0.00 sec) mysql> select * from test; +------+------+------+ | id | name | addr | +------+------+------+ | 1 | a | b | | 1 | a | b | | 1 | a | b | | 1 | NULL | b | +------+------+------+ 4 rows in set (0.00 sec) mysql> insert into test(id,addr) values (1,null); ERROR 1048 (23000): Column 'addr' cannot be null mysql>