1、自动编号(AUTO_INCREMENT)，必须与主键组合使用
默认情况下，起始值为1，增量也为1。
2、主键(PRIMARY KEY)
每张数据表只能存在一个主键
主键保证记录的唯一性
主键自动为NOT NULL
3、AUTO_INCREMENT必须与主键一起使用，主键不一定必须与AUTO_INCREMENT一起使用。
mysql> CREATE TABLE tb3(
-> id SMALLINT UNSIGNEDAUTO_INCREMENT PRIMARY KEY,
-> username VARCHAR(30) NOT NULL
-> );
Query OK, 0 rows affected (0.35 sec)
mysql>
mysql> SHOW COLUMNS FROM tb3; #查看数据表结构
+----------+----------------------+------+-----+---------+----------------+
| Field | Type | Null | Key | Default | Extra|
+----------+----------------------+------+-----+---------+----------------+
| id| smallint(5) unsigned | NO | PRI | NULL | auto_increment |
| username | varchar(30) | NO | | NULL |
+----------+----------------------+------+-----+---------+----------------+
2 rows in set (0.14 sec)
mysql> INSERT tb3(useRname) VALUES('JOHN');
Query OK, 1 row affected (0.04 sec)
mysql> INSERT tb3(useRname) VALUES('Tom');
Query OK, 1 row affected (0.04 sec)
mysql> INSERT tb3(useRname) VALUES('Rose');
Query OK, 1 row affected (0.02 sec)
mysql> INSERT tb3(useRname) VALUES('Dimi tar');
Query OK, 1 row affected (0.04 sec)
mysql> SELECT * FROM tb3;
+----+----------+
| id | username |
+----+----------+
| 1 | JOHN |
| 2 | Tom |
| 3 | Rose |
| 4 | Dimi tar |
+----+----------+
4、创建一张表只有主键，没有AUTO_INCREMENT
mysql> CREATE TABLE tb4(
-> id SMALLINT UNSIGNED PRIMARY KEY,
-> usename VARCHAR(20) NOT NULL
-> );
Query OK, 0 rows affected (0.22 sec)
mysql>
mysql> SHOW COLUMNS FROM tb4;
+---------+----------------------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+---------+----------------------+------+-----+---------+-------+
|id| smallint(5) unsigned | NO | PRI | NULL ||
| usename | varchar(20) | NO | | NULL ||
+---------+----------------------+------+-----+---------+-------+
2 rows in set (0.07 sec)
主键允许赋值，但不允许存在两个相同的值
mysql> INSERT tb4 VALUES(4, 'Tom');
Query OK, 1 row affected (0.03 sec)
mysql> INSERT tb4 VALUES(22, 'Tom');
Query OK, 1 row affected (0.05 sec)
mysql> SELECT * FROM tb4;
+----+---------+
| id | usename|
+----+---------+
| 4 | Tom |
|22 | Tom |
+----+---------+
2 rows in set (0.00 sec)
mysql> INSERT tb4 VALUES(22, 'Li');
ERROR 1062 (23000): Duplicate entry '22' for key 'PRIMARY'
