---非空約束
---修飾的欄位不能為空NULL

mysql> create table user9(
    -> id int,
    -> name varchar(20) not null
    -> );
Query OK, 0 rows affected (0.01 sec)

mysql> desc user9;
+-------+-------------+------+-----+---------+-------+
| Field | Type        | Null | Key | Default | Extra |
+-------+-------------+------+-----+---------+-------+
 

| id    | int         | YES  |     | NULL    |       |
| name  | varchar(20) | NO   |     | NULL    |       |
+-------+-------------+------+-----+---------+-------+
2 rows in set (0.00 sec)

mysql> 

mysql> insert into user9 (id) values (1);
ERROR 1364 (HY000): Field 'name' doesn't have a default value
mysql> 

mysql> insert into user9 (id) values (1);
ERROR 1364 (HY000): Field 
 
'name' doesn't have a default value
mysql> insert into user9 values(1,"zhangsan");
Query OK, 1 row affected (0.00 sec)

mysql> desc user9;
+-------+-------------+------+-----+---------+-------+
| Field | Type        | Null | Key | Default | Extra |
+-------+-------------+------+-----+---------+-------+
 

| id    | int         | YES  |     | NULL    |       |
| name  | varchar(20) | NO   |     | NULL    |       |
+-------+-------------+------+-----+---------+-------+
2 rows in set (0.00 sec)

mysql> select * from user9;
+------+----------+
| id   | name     |
+------+----------+
|    1 | zhangsan |
+------+----------+
1 row in set (0.00 sec)

mysql> 


---再舉個例子：

mysql> create table user66(
    -> id int not null,
    -> name varchar(20)
    -> );
Query OK, 0 rows affected (0.01 sec)

mysql> desc user66;
+-------+-------------+------+-----+---------+-------+
| Field | Type        | Null | Key | Default | Extra |
+-------+-------------+------+-----+---------+-------+
| id    | int         | NO   |     | NULL    |       |
| name  | varchar(20) | YES  |     | NULL    |       |
+-------+-------------+------+-----+---------+-------+
2 rows in set (0.01 sec)
mysql> insert into user66(name) values("zhangsan");
ERROR 1364 (HY000): Field 'id' doesn't have a default value
mysql>

mysql> insert into user66(id) values("9");
Query OK, 1 row affected (0.00 sec)

mysql> select * from user66;
+----+------+
| id | name |
+----+------+
|  9 | NULL |
+----+------+
1 row in set (0.00 sec)

mysql>