【MYSQL】UNIQUE KEY

唯一索引

创建唯一索引的目的不是为了提高访问速度，而只是为了避免数据出现重复。唯一索引可以有多个但索引列的值必须唯一，索引列的值允许有空值。如果能确定某个数据列将只包含彼此各不相同的值，在为这个数据列创建索引的时候就应该使用关键字UNIQUE

索引列的值必须唯一，但允许有空值。如果是组合索引，则列值的组合必须唯一。它有以下几种创建方式：

（1）创建索引：CREATE UNIQUE INDEX indexName ON tableName(tableColumns(length))
（2）修改表结构：ALTER tableName ADD UNIQUE [indexName] ON (tableColumns(length))
（3）创建表的时候直接指定：CREATE TABLE tableName ( [...], UNIQUE [indexName] (tableColumns(length));

CREATE table users (
 uname VARCHAR(50) PRIMARY KEY,
 uint VARCHAR(50) UNIQUE
)

mysql> desc users;
+-------+-------------+------+-----+---------+-------+
| Field | Type        | Null | Key | Default | Extra |
+-------+-------------+------+-----+---------+-------+
| uname | varchar(50) | NO   | PRI | NULL    |       |
| uint  | varchar(50) | YES  | UNI | NULL    |       |
+-------+-------------+------+-----+---------+-------+
2 rows in set (0.00 sec)

mysql> insert into users values('li','basket');
Query OK, 1 row affected (0.00 sec)

mysql> insert into users values('ti','basket');
ERROR 1062 (23000): Duplicate entry 'basket' for key 'uint'


mysql> insert into users values('ti','FOOT');
Query OK, 1 row affected (0.00 sec)

mysql> select * from users;
+-------+--------+
| uname | uint |
+-------+--------+
| li | basket |
| ti | FOOT |
+-------+--------+
2 rows in set (0.00 sec)

 

Q

CREATE table users2 (
 uname VARCHAR(50) PRIMARY KEY,
 uint VARCHAR(50),
 UNIQUE KEY `uint_uniq` (`uint`)  
)

mysql> desc users2;
+-------+-------------+------+-----+---------+-------+
| Field | Type        | Null | Key | Default | Extra |
+-------+-------------+------+-----+---------+-------+
| uname | varchar(50) | NO   | PRI | NULL    |       |
| uint  | varchar(50) | YES  | UNI | NULL    |       |
+-------+-------------+------+-----+---------+-------+
2 rows in set (0.00 sec)

Q

mysql> desc users3;
+-------+-------------+------+-----+---------+-------+
| Field | Type        | Null | Key | Default | Extra |
+-------+-------------+------+-----+---------+-------+
| uname | varchar(50) | NO   | PRI | NULL    |       |
| uint  | varchar(50) | YES  |     | NULL    |       |
+-------+-------------+------+-----+---------+-------+
2 rows in set (0.01 sec)

mysql> 
mysql> CREATE UNIQUE INDEX users3_uniqkey ON users3(uint); 
Query OK, 0 rows affected (0.11 sec)
Records: 0  Duplicates: 0  Warnings: 0

mysql> desc users3;
+-------+-------------+------+-----+---------+-------+
| Field | Type        | Null | Key | Default | Extra |
+-------+-------------+------+-----+---------+-------+
| uname | varchar(50) | NO   | PRI | NULL    |       |
| uint  | varchar(50) | YES  | UNI | NULL    |       |
+-------+-------------+------+-----+---------+-------+
2 rows in set (0.01 sec)
mysql>  ALTER TABLE users3 DROP INDEX users3_uniqkey;
Query OK, 0 rows affected (0.03 sec)
Records: 0  Duplicates: 0  Warnings: 0

mysql> desc users3;
+-------+-------------+------+-----+---------+-------+
| Field | Type        | Null | Key | Default | Extra |
+-------+-------------+------+-----+---------+-------+
| uname | varchar(50) | NO   | PRI | NULL    |       |
| uint  | varchar(50) | YES  |     | NULL    |       |
+-------+-------------+------+-----+---------+-------+
2 rows in set (0.01 sec)

 Q

CREATE table users4 (
 uname VARCHAR(50) PRIMARY KEY,
 uint VARCHAR(50)
)

mysql> desc users4;
+-------+-------------+------+-----+---------+-------+
| Field | Type        | Null | Key | Default | Extra |
+-------+-------------+------+-----+---------+-------+
| uname | varchar(50) | NO   | PRI | NULL    |       |
| uint  | varchar(50) | YES  |     | NULL    |       |
+-------+-------------+------+-----+---------+-------+
2 rows in set (0.01 sec)

mysql> 
mysql> alter table users4  add unique index(uint);
Query OK, 0 rows affected (0.10 sec)
Records: 0  Duplicates: 0  Warnings: 0

mysql> desc users4;
+-------+-------------+------+-----+---------+-------+
| Field | Type        | Null | Key | Default | Extra |
+-------+-------------+------+-----+---------+-------+
| uname | varchar(50) | NO   | PRI | NULL    |       |
| uint  | varchar(50) | YES  | UNI | NULL    |       |
+-------+-------------+------+-----+---------+-------+
2 rows in set (0.00 sec)