mysql UNIQUE索引的使用方法详解

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: 0

mysql>
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;

补充：数据库,mysql教程