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 UNSIGNED AUTO_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'