mysql教程 UNIQUE索引的使用方法详解
语法：
UNIQUE KEY `test` (`xref_key`,`xref_dbname`)
UNIQUE KEY `xref_key` (`xref_key`,`xref_dbname`)
上述语法检查xref_key与xref_dbname的组合是否是唯一值，可以设定多个字段的组
合。其中，test是索引表中的字段名称。
该方法适合用来解决多个字段相互依赖，且必须满足唯一性的条件的情况。在经常
新增或删除资料表中auto_increment的primary key会造成大量的断层，这类易变动的资料表
使用auto_increment并不是一个很适合的索引值，所以可以采取unique key来处理。
实例字段值不重复
CREATE TABLE `test1` (
`id` int(11) NOT NULL auto_increment,
`name` varchar(11) default NULL,
PRIMARY KEY (`id`),
UNIQUE KEY (`name`)
);实例
mysql> CREATE TABLE Employee(
-> id int,
-> first_name VARCHAR(15),
-> last_name VARCHAR(15),
-> start_date DATE,
-> end_date DATE,
-> salary FLOAT(8,2),
-> city VARCHAR(10),
-> description VARCHAR(15)
-> );
Query OK, 0 rows affected (0.03 sec)mysql>
mysql>
mysql> insert into Employee(id,first_name, last_name, start_date, end_Date,salary, City, Description)
-> values (1,'Jason', 'Martin', '19960725', '20060725',1234.56, 'Toronto', 'Programmer');
Query OK, 1 row affected (0.00 sec)mysql>
mysql> insert into Employee(id,first_name, last_name, start_date, end_Date,salary, City, Description)
-> values(2,'Alison', 'Mathews', '19760321', '19860221',6661.78, 'Vancouver','Tester');
Query OK, 1 row affected (0.00 sec)mysql>
mysql> insert into Employee(id,first_name, last_name, start_date, end_Date,salary, City, Description)
-> values(3,'James', 'Smith', '19781212', '19900315',6544.78, 'Vancouver','Tester');
Query OK, 1 row affected (0.00 sec)mysql>
mysql> insert into Employee(id,first_name, last_name, start_date, end_Date,salary, City, Description)
-> values(4,'Celia', 'Rice', '19821024', '19990421',2344.78, 'Vancouver','Manager');
Query OK, 1 row affected (0.01 sec)mysql>
mysql> insert into Employee(id,first_name, last_name, start_date, end_Date,salary, City, Description)
-> values(5,'Robert', 'Black', '19840115', '19980808',2334.78, 'Vancouver','Tester');
Query OK, 1 row affected (0.00 sec)mysql>
mysql> insert into Employee(id,first_name, last_name, start_date, end_Date,salary, City, Description)
-> values(6,'Linda', 'Green', '19870730', '19960104',4322.78,'New York', 'Tester');
Query OK, 1 row affected (0.00 sec)mysql>
mysql> insert into Employee(id,first_name, last_name, start_date, end_Date,salary, City, Description)
-> values(7,'David', 'Larry', '19901231', '19980212',7897.78,'New York', 'Manager');
Query OK, 1 row affected (0.00 sec)mysql>
mysql> insert into Employee(id,first_name, last_name, start_date, end_Date,salary, City, Description)
-> values(8,'James', 'Cat', '19960917', '20020415',1232.78,'Vancouver', 'Tester');
Query OK, 1 row affected (0.02 sec)mysql>
mysql> select * from Employee;
+------+------------+-----------+------------+------------+---------+-----------+-------------+
| id | first_name | last_name | start_date | end_date | salary | city |description |
+------+------------+-----------+------------+------------+---------+-----------+-------------+
| 1 | Jason | Martin | 1996-07-25 | 2006-07-25 | 1234.56 | Toronto |Programmer |
| 2 | Alison | Mathews | 1976-03-21 | 1986-02-21 | 6661.78 | Vancouver |Tester |
| 3 | James | Smith | 1978-12-12 | 1990-03-15 | 6544.78 | Vancouver |Tester |
| 4 | Celia | Rice | 1982-10-24 | 1999-04-21 | 2344.78 | Vancouver |Manager |
| 5 | Robert | Black | 1984-01-15 | 1998-08-08 | 2334.78 | Vancouver |Tester |
| 6 | Linda | Green | 1987-07-30 | 1996-01-04 | 4322.78 | New York |Tester |
| 7 | David | Larry | 1990-12-31 | 1998-02-12 | 7897.78 | New York |Manager |
| 8 | James | Cat | 1996-09-17 | 2002-04-15 | 1232.78 | Vancouver |Tester |
+------+------------+-----------+------------+------------+---------+-----------+-------------+
8 rows in set (0.00 sec)mysql>
mysql>
mysql>
mysql> ALTER TABLE Employee ADD UNIQUE address_index (city);
ERROR 1062 (23000): Duplicate entry 'Vancouver' for key 1
mysql> ALTER TABLE Employee ADD UNIQUE lastn_index (last_name);
Query OK, 8 rows affected (0.02 sec)
Records: 8 Duplicates: 0 Warnings: 0mysql>
mysql> desc Employee;
+-------------+-------------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+-------------+-------------+------+-----+---------+-------+
| id | int(11) | YES | | NULL | |
| first_name | varchar(15) | YES | | NULL | |
| last_name | varchar(15) | YES | UNI | NULL | |
| start_date | date | YES | | NULL | |
| end_date | date | YES | | NULL | |
| salary | float(8,2) | YES | | NULL | |
| city | varchar(10) | YES | | NULL | |
| description | varchar(15) | YES | | NULL | |
+-------------+-------------+------+-----+---------+-------+
8 rows in set (0.03 sec)如果要给同一表两个字段增加unique索引，那么看实例
mysql>
mysql> CREATE TABLE myTable
-> (
-> OrderID SMALLINT UNSIGNED NOT NULL,
-> ModelID SMALLINT UNSIGNED NOT NULL,
-> Description VARCHAR(40),
-> PRIMARY KEY (OrderID),
-> UNIQUE (OrderID, ModelID)
-> );
Query OK, 0 rows affected (0.05 sec)mysql>
mysql> desc myTable;
+-------------+----------------------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+-------------+----------------------+------+-----+---------+-------+
| OrderID | smallint(5) unsigned | NO | PRI | | |
| ModelID | smallint(5) unsigned | NO | | | |
| Description | varchar(40) | YES | | NULL | |
+-------------+----------------------+------+-----+---------+-------+
3 rows in set (0.02 sec)下面如果我要删除索引怎么办，如下实例
alter table fuinfo drop index email;