常用的索引分为下面几种:key:普通索引unique key:唯一索引primary key:主键索引full text:全文索引实例操作：创建表t8添加索引:mysql> create table t8 ( id int, name char(10), email char(20),key name(name),unique key(email) );Query OK, 0 rows affected (0.01 sec)mysql> desc t8;+-------+----------+------+-----+---------+-------+| Field | Type | Null | Key | Default | Extra |+-------+----------+------+-----+---------+-------+| id | int(11) | YES | | NULL | || name | char(10) | YES | MUL | NULL | || email | char(20) | YES | UNI | NULL | |+-------+----------+------+-----+---------+-------+3 rows in set (0.00 sec)插入数据:mysql> insert into t8 values ("1","lisi","lisi@email.com") -> ;Query OK, 1 row affected (0.01 sec)查询数据:mysql> select * from t8;+------+------+----------------+| id | name | email |+------+------+----------------+| 1 | lisi | lisi@email.com |+------+------+----------------+1 row in set (0.00 sec)#unique key唯一性,lisi@email.com邮箱已经存在将不允许相同.mysql> insert into t8 values ("2","lisi","lisi@email.com") -> ;ERROR 1062 (23000): Duplicate entry 'lisi@email.com' for key 'email'#创建表t9，主键key。mysql> create table t9 ( id int, name char(10), email char(20),primary key (id),key id (name),unique key(email) );Query OK, 0 rows affected (0.04 sec)mysql> desc t9;+-------+----------+------+-----+---------+-------+| Field | Type | Null | Key | Default | Extra |+-------+----------+------+-----+---------+-------+| id | int(11) | NO | PRI | 0 | || name | char(10) | YES | MUL | NULL | || email | char(20) | YES | UNI | NULL | |+-------+----------+------+-----+---------+-------+3 rows in set (0.00 sec)##索引的长度:建索引时，可以只索引列的前一部分的内容,比如前10个字符.如 key id(email(10))#索引email长度为10个字符，并不是只允许表中email列插入长度为10个字符,而是说索引引用时只引用emial列前面10个字符.mysql> create table t10 ( id int, name char(10), email char(20),primary key (id),key id (email(10)),unique key(email) );Query OK, 0 rows affected (0.02 sec)mysql> desc t10;+-------+----------+------+-----+---------+-------+| Field | Type | Null | Key | Default | Extra |+-------+----------+------+-----+---------+-------+| id | int(11) | NO | PRI | 0 | || name | char(10) | YES | | NULL | || email | char(20) | YES | UNI | NULL | |+-------+----------+------+-----+---------+-------+3 rows in set (0.00 sec)#插入超过10个字符长度的email，发现也是可以的.mysql> insert into t10 values (1,"lisi","sqweqeqeq@email.com");Query OK, 1 row affected (0.01 sec)多列索引:把2个或者多列的值，看成一个整体，然后建立索引.mysql> create table t11 ( -> xing char(2), -> ming char(10), -> key xm(xing,ming) -> );Query OK, 0 rows affected (0.02 sec)mysql> desc t11;+-------+----------+------+-----+---------+-------+| Field | Type | Null | Key | Default | Extra |+-------+----------+------+-----+---------+-------+| xing | char(2) | YES | MUL | NULL | || ming | char(10) | YES | | NULL | |+-------+----------+------+-----+---------+-------+2 rows in set (0.00 sec)mysql> insert into t11 values ("李","世民");Query OK, 1 row affected (0.00 sec)mysql> select * from t11;+------+--------+| xing | ming |+------+--------+| 李 | 世民 |+------+--------+1 row in set (0.00 sec)mysql> show index from t11 \G;*************************** 1. row *************************** Table: t11 Non_unique: 1 Key_name: xm Seq_in_index: 1 Column_name: xing Collation: A Cardinality: 1 Sub_part: NULL Packed: NULL Null: YES Index_type: BTREE Comment: Index_comment: *************************** 2. row *************************** Table: t11 Non_unique: 1 Key_name: xm Seq_in_index: 2 Column_name: ming Collation: A Cardinality: 1 Sub_part: NULL Packed: NULL Null: YES Index_type: BTREE Comment: Index_comment: 2 rows in set (0.00 sec)ERROR: No query specified#多列索引的查询。mysql> select * from t11 where xing="李" and ming="世民";+------+--------+| xing | ming |+------+--------+| 李 | 世民 |+------+--------+1 row in set (0.01 sec)查看查询所能使用的索引：explainmysql> explain select * from t11 where xing="李" and ming="世民";+----+-------------+-------+------+---------------+------+---------+-------------+------+--------------------------+| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |+----+-------------+-------+------+---------------+------+---------+-------------+------+--------------------------+| 1 | SIMPLE | t11 | ref | xm | xm | 38 | const,const | 1 | Using where; Using index |+----+-------------+-------+------+---------------+------+---------+-------------+------+--------------------------+1 row in set (0.00 sec)mysql> explain select * from t11 where xing="李";+----+-------------+-------+------+---------------+------+---------+-------+------+--------------------------+| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |+----+-------------+-------+------+---------------+------+---------+-------+------+--------------------------+| 1 | SIMPLE | t11 | ref | xm | xm | 7 | const | 1 | Using where; Using index |+----+-------------+-------+------+---------------+------+---------+-------+------+--------------------------+1 row in set (0.00 sec)mysql> explain select * from t11 where ming="世民";+----+-------------+-------+-------+---------------+------+---------+------+------+--------------------------+| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |+----+-------------+-------+-------+---------------+------+---------+------+------+--------------------------+| 1 | SIMPLE | t11 | index | NULL | xm | 38 | NULL | 1 | Using where; Using index |+----+-------------+-------+-------+---------------+------+---------+------+------+--------------------------+1 row in set (0.00 sec)mysql> explain select * from t11 where ming="世民" and xing="李";+----+-------------+-------+------+---------------+------+---------+-------------+------+--------------------------+| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |+----+-------------+-------+------+---------------+------+---------+-------------+------+--------------------------+| 1 | SIMPLE | t11 | ref | xm | xm | 38 | const,const | 1 | Using where; Using index |+----+-------------+-------+------+---------------+------+---------+-------------+------+--------------------------+1 row in set (0.00 sec)冗余索引:在某个列上，可能存在多个索引.比如xm(xing,ming) m(ming).目的在于加快查询速度.mysql> create table t12 ( xing char(2), ming char(10), key xm(xing,ming),key m(ming) );Query OK, 0 rows affected (0.02 sec)mysql> show index from t12;+-------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+| Table | Non_unique | Key_name | Seq_in_index | Column_name | Collation | Cardinality | Sub_part | Packed | Null | Index_type | Comment | Index_comment |+-------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+| t12 | 1 | xm | 1 | xing | A | 0 | NULL | NULL | YES | BTREE | | || t12 | 1 | xm | 2 | ming | A | 0 | NULL | NULL | YES | BTREE | | || t12 | 1 | m | 1 | ming | A | 0 | NULL | NULL | YES | BTREE | | |+-------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+3 rows in set (0.00 sec)mysql> show index from t12 \G;*************************** 1. row *************************** Table: t12 Non_unique: 1 Key_name: xm Seq_in_index: 1 Column_name: xing Collation: A Cardinality: 0 Sub_part: NULL Packed: NULL Null: YES Index_type: BTREE Comment: Index_comment: *************************** 2. row *************************** Table: t12 Non_unique: 1 Key_name: xm Seq_in_index: 2 Column_name: ming Collation: A Cardinality: 0 Sub_part: NULL Packed: NULL Null: YES Index_type: BTREE Comment: Index_comment: *************************** 3. row *************************** Table: t12 Non_unique: 1 Key_name: m Seq_in_index: 1 Column_name: ming Collation: A Cardinality: 0 Sub_part: NULL Packed: NULL Null: YES Index_type: BTREE Comment: Index_comment: 3 rows in set (0.00 sec)ERROR: No query specified#删除索引.语法：alter table 表名 drop index 索引名drop index 索引名 on 表名#删除索引mmysql> alter table t12 drop index m;Query OK, 0 rows affected (0.02 sec)Records: 0 Duplicates: 0 Warnings: 0mysql> drop index xm on t12;Query OK, 0 rows affected (0.01 sec)Records: 0 Duplicates: 0 Warnings: 0mysql> show index from t12 \G;Empty set (0.00 sec)ERROR: No query specified添加索引alter table 表名 add [index/unique] 索引名;mysql> alter table t12 add index xm(xing,ming);Query OK, 0 rows affected (0.02 sec)Records: 0 Duplicates: 0 Warnings: 0mysql> alter table t12 add unique m(ming);Query OK, 0 rows affected (0.01 sec)Records: 0 Duplicates: 0 Warnings: 0mysql> show create table t12;+-------+--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+| Table | Create Table |+-------+--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+| t12 | CREATE TABLE `t12` ( `xing` char(2) DEFAULT NULL, `ming` char(10) DEFAULT NULL, UNIQUE KEY `m` (`ming`), KEY `xm` (`xing`,`ming`)) ENGINE=InnoDB DEFAULT CHARSET=utf8 |+-------+--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+1 row in set (0.00 sec)添加主键索引mysql> alter table t12 add primary key(xing);Query OK, 0 rows affected (0.02 sec)Records: 0 Duplicates: 0 Warnings: 0删除主键索引mysql> alter table t12 drop primary key;Query OK, 0 rows affected (0.02 sec)Records: 0 Duplicates: 0 Warnings: 0