自动编号AUTO_INCREMENT

自动编号必须和主键组合使用；在默认情况下，起始值为1，每次的增量为1；
it must be defined as a key ；
报错为自动编号的字段必须定义为主键；

主键自动为NOT NULL 主键约束 保证记录的唯一性  每个数据表只能存在一个主键 ；
AUTO_INCREMENT必须和主键组合使用  但是主键未必和AUTO_INCREMENT相即使用 ；

mysql> CREATE TABLE five(
    -> id SMALLINT UNSIGNED AUTO_INCREMENT PRIMARY KEY,
    -> username VARCHAR(30) NOT NULL
    -> );
Query OK, 0 rows affected (0.02 sec)

mysql> SHOW COLUMNS FROM five;                            Extra字段无需赋值；
+----------+----------------------+------+-----+---------+----------------+
| Field    | Type                 | Null | Key | Default | Extra          |
+----------+----------------------+------+-----+---------+----------------+
| id       | smallint(5) unsigned | NO   | PRI | NULL    | auto_increment |
| username | varchar(30)          | NO   |     | NULL    |                |
+----------+----------------------+------+-----+---------+----------------+
2 rows in set (0.03 sec)
mysql> INSERT five(username)VALUES('Tom');                给部分字段赋值 不能省去小括号；
Query OK, 1 row affected (0.00 sec)

mysql> INSERT five(username)VALUES('Daming');
Query OK, 1 row affected (0.00 sec)

mysql> INSERT five(username)VALUES('Feiyu');
Query OK, 1 row affected (0.00 sec)

mysql> INSERT five(username)VALUES('Jean');
Query OK, 1 row affected (0.01 sec)

mysql> SELECT * FROM five;
+----+----------+
| id | username |
+----+----------+   AUTO_INCREMENT必须定义为主键 相反主键未必和AUTO_INCREMENT相即使用 ；
|  1 | Tom      |
|  2 | Daming   |
|  3 | Feiyu    |
|  4 | Jean     |
+----+----------+
4 rows in set (0.00 sec)    

mysql> CREATE TABLE six(
    -> id SMALLINT UNSIGNED PRIMARY KEY,
    -> username VARCHAR(30) NOT NULL
    -> );
Query OK, 0 rows affected (0.00 sec) 

mysql> SHOW COLUMNS FROM six;
+----------+----------------------+------+-----+---------+-------+
| Field    | Type                 | Null | Key | Default | Extra |
+----------+----------------------+------+-----+---------+-------+
| id       | smallint(5) unsigned | NO   | PRI | NULL    |       |
| username | varchar(30)          | NO   |     | NULL    |       |
+----------+----------------------+------+-----+---------+-------+
2 rows in set (0.02 sec)  

mysql> INSERT six VALUES(20,'Feiyu');
Query OK, 1 row affected (0.00 sec)

mysql> INSERT six VALUES(2,'Feiyu');
Query OK, 1 row affected (0.00 sec)

mysql> INSERT six VALUES(21,'Feiyu');
Query OK, 1 row affected (0.00 sec)

mysql> SELECT * FROM six;                         主键字段允许赋值；
+----+----------+
| id | username |
+----+----------+
| 20 | Feiyu    |
|  2 | Feiyu    |
| 21 | Feiyu    |
+----+----------+
3 rows in set (0.00 sec)      
                              
mysql> INSERT six VALUES(21,'Feiyu');
ERROR 1062 (23000): Duplicate entry '21' for key 'PRIMARY'          
mysql>      Duplicate entry报错为不能 使用重复id  
            可以明白AUTO_INCREMENT必须和PRIMARY一同使用 相反PRIMARY 不一定和 AUTO_INCREMENT一起使用；