UNIQUE KEY 唯一约束

唯一约束可以保证记录唯一性 约束字段可以为空值NULL 每个表可以有多个UNIQUE约束 但是每个数据表只能有一个 PRIMARY KEY 约束；
mysql> CREATE TABLE SEVEN(
    -> id SMALLINT UNSIGNED AUTO_INCREMENT PRIMARY KEY,
    -> username VARCHAR(20) NOT NULL UNIQUE KEY,
    -> age TINYINT UNSIGNED
    -> );
Query OK, 0 rows affected (0.01 sec)

mysql> SHOW COLUMNS FROM seven;
+----------+----------------------+------+-----+---------+----------------+
| Field    | Type                 | Null | Key | Default | Extra          |
+----------+----------------------+------+-----+---------+----------------+
| id       | smallint(5) unsigned | NO   | PRI | NULL    | auto_increment |
| username | varchar(20)          | NO   | UNI | NULL    |                |
| age      | tinyint(3) unsigned  | YES  |     | NULL    |                |
+----------+----------------------+------+-----+---------+----------------+
3 rows in set (0.03 sec)
mysql> INSERT seven(username,age)VALUES("Tom",21);
Query OK, 1 row affected (0.00 sec)

mysql> INSERT seven(username,age)VALUES("Tom",21);
ERROR 1062 (23000): Duplicate entry 'Tom' for key 'username'    唯一性开始报错；
mysql>