为表的字段创建索引
索引就像书的目录一样，如果在字段上建立索引，那么以索引为条件时可以加快查询数据的速度。
1 创建主键索引
查询数据库的内容，按主键查询是最快的，每个表只能有一个主键，但是可以有多个普通索引列，主键列要求所有内容必须唯一，而索引列不要求内容唯一。
我们无论建立主键索引还是普通索引，都要在表的对应列上创建，可以对单列创建索引也可以对多列创建索引
建立主键索方法：
1.在创建表时，可以增加建立主键索引语句
[email protected] 04:0932->create table student(
-> id int(4) not null AUTO_INCREMENT,
-> name char(20) not null,
-> age tinyint(2) NOT NULL default ‘0‘,
-> dept varchar(16) default NULL,
-> primary key(id),
-> KEY index_name(name)
-> );提示：
a.AUTO_INCREMENT 自增
b.Primary key(id) 主键
c.KEY index_name(name) name 字段普通索引
操作演示
[email protected] 04:2146->drop table student;
Query OK, 0 rows affected (0.00 sec)
[email protected] 04:2333->create table student(
-> id int(4) not null AUTO_INCREMENT,
-> name char(20) not null,
-> age tinyint(2) NOT NULL default ‘0‘,
-> dept varchar(16) default NULL,
-> primary key(id),
-> KEY index_name(name)
-> );
Query OK, 0 rows affected (0.00 sec)
[email protected] 04:2609->desc student;
+-------+-------------+------+-----+---------+----------------+
| Field | Type| Null | Key | Default | Extra |
+-------+-------------+------+-----+---------+----------------+
| id| int(4) | NO | PRI | NULL| auto_increment |
| name | char(20)| NO | MUL | NULL||
| age | tinyint(2) | NO | | 0 ||
| dept | varchar(16) | YES | | NULL||
+-------+-------------+------+-----+---------+----------------+
4 rows in set (0.00 sec)提示：PRI为主键的标识，MUL为普通索引的标识。
2.建立表之后通过alter命令增加主键索引（不推荐这种做法）
a.主键列不能重复创建，必须先删除上面的配置
[email protected] 04:3716->alter table student drop primary key;b.建表时忘记加主键了利用alter命令增加id列为自增主键列
[email protected] 04:5948->alter table student change id id int primary key auto_increment;
Query OK, 0 rows affected (0.00 sec)
Records: 0 Duplicates: 0 Warnings: 0
[email protected] 05:0324->desc student;
+-------+-------------+------+-----+---------+----------------+
| Field | Type| Null | Key | Default | Extra |
+-------+-------------+------+-----+---------+----------------+
| id| int(11) | NO | PRI | NULL| auto_increment |
| name | char(20)| NO | MUL | NULL||
| age | tinyint(2) | NO | | 0 ||
| dept | varchar(16) | YES | | NULL||
+-------+-------------+------+-----+---------+----------------+
4 rows in set (0.00 sec)提示：只有int类型且为primary key才可以使用auto_increment。
9.9.7.2 创建普通索引
1.在建表时，可以增加建立普通索引列的语句如下：
[email protected] 04:0932->create table student(
-> id int(4) not null AUTO_INCREMENT,
-> name char(20) not null,
-> age tinyint(2) NOT NULL default ‘0‘,
-> dept varchar(16) default NULL,
-> primary key(id),
-> KEY index_name(name)
-> );提示：
a.KEY index_name(name)name 字段普通索引
操作演示：
[email protected] 04:2146->drop table student;
Query OK, 0 rows affected (0.00 sec)
[email protected] 04:2333->create table student(
-> id int(4) not null AUTO_INCREMENT,
-> name char(20) not null,
-> age tinyint(2) NOT NULL default ‘0‘,
-> dept varchar(16) default NULL,
-> primary key(id),
-> KEY index_name(name)
-> );
Query OK, 0 rows affected (0.00 sec)
[email protected] 04:2609->desc student;
+-------+-------------+------+-----+---------+----------------+
| Field | Type| Null | Key | Default | Extra |
+-------+-------------+------+-----+---------+----------------+
| id| int(4) | NO | PRI | NULL| auto_increment |
| name | char(20)| NO | MUL | NULL||
| age | tinyint(2) | NO | | 0 ||
| dept | varchar(16) | YES | | NULL||
+-------+-------------+------+-----+---------+----------------+
4 rows in set (0.00 sec)2.建表后利用alter增加普通索引
删除建表时创建的index_name索引
[email protected] 05:0333->alter table student drop index index_name;
Query OK, 0 rows affected (0.01 sec)
Records: 0 Duplicates: 0 Warnings: 0
[email protected] 06:0419->desc student;
+-------+-------------+------+-----+---------+----------------+
| Field | Type| Null | Key | Default | Extra |
+-------+-------------+------+-----+---------+----------------+
| id| int(11) | NO | PRI | NULL| auto_increment |
| name | char(20)| NO | | NULL||
| age | tinyint(2) | NO | | 0 ||
| dept | varchar(16) | YES | | NULL||
+-------+-------------+------+-----+---------+----------------+
4 rows in set (0.00 sec)在name列上添加索引，索引名为index_name
[email protected] 06:0431->alter table student add index index_name(name);
Query OK, 0 rows affected (0.00 sec)
Records: 0 Duplicates: 0 Warnings: 0
[email protected] 06:0456->desc student;
+-------+-------------+------+-----+---------+----------------+
| Field | Type| Null | Key | Default | Extra |
+-------+-------------+------+-----+---------+----------------+
| id| int(11) | NO | PRI | NULL| auto_increment |
| name | char(20)| NO | MUL | NULL||
| age | tinyint(2) | NO | | 0 ||
| dept | varchar(16) | YES | | NULL||
+-------+-------------+------+-----+---------+----------------+
4 rows in set (0.00 sec)3 对字段的前n个字符创建普通索引
当遇到表中比较大的列时，列内容的前n个字符在所有内容中已经接近唯一时，这时可以对列的前n个字符建立索引而无需对整个列建立索引，这样可以节省创建索引占用的系统空间，以及降低和更新维护消耗的系统资源。
对字段的前n个字符创建索引的语法：
Create index index_name on test(name(8)); 条件列前n个字符创建索引
实战操作：
[email protected] 06:0838->create index index_dept on student(dept(8));
Query OK, 0 rows affected (0.19 sec)
Records: 0 Duplicates: 0 Warnings: 0
[email protected] 06:3650->desc student;
+-------+-------------+------+-----+---------+----------------+
| Field | Type| Null | Key | Default | Extra |
+-------+-------------+------+-----+---------+----------------+
| id| int(11) | NO | PRI | NULL| auto_increment |
| name | char(20)| NO | MUL | NULL||
| age | tinyint(2) | NO | | 0 ||
| dept | varchar(16) | YES | MUL | NULL||
+-------+-------------+------+-----+---------+----------------+
4 rows in set (0.00 sec)
[email protected] 06:3847->show index from student\G
*************************** 1. row ***************************
Table: student
Non_unique: 0
Key_name: PRIMARY
Seq_in_index: 1
Column_name: id
Collation: A
Cardinality: 0
Sub_part: NULL
Packed: NULL
Null:
Index_type: BTREE
Comment:
*************************** 2. row ***************************
Table: student
Non_unique: 1
Key_name: index_name
Seq_in_index: 1
Column_name: name
Collation: A
Cardinality: NULL
Sub_part: NULL
Packed: NULL
Null:
Index_type: BTREE
Comment:
*************************** 3. row ***************************
Table: student
Non_unique: 1
Key_name: index_dept
Seq_in_index: 1
Column_name: dept
Collation: A
Cardinality: NULL
Sub_part: 8
Packed: NULL
Null: YES
Index_type: BTREE
Comment:
3 rows in set (0.00 sec)4 为表的多个字段创建联合索引
如果查询的条件是多列时，我们可以为多个查询的列创建联合索引，甚至可以为多列的前n个字符创建联合索引，实战演示如下：
为多个列创建索引
[email protected] 06:4019->create index ind_name_dept on student(name,dept);
Query OK, 0 rows affected (0.01 sec)
Records: 0 Duplicates: 0 Warnings: 0
[email protected] 06:4726->show index from student\G
*************************** 1. row ***************************
Table: student
Non_unique: 0
Key_name: PRIMARY
Seq_in_index: 1
Column_name: id
Collation: A
Cardinality: 0
Sub_part: NULL
Packed: NULL
Null:
Index_type: BTREE
Comment:
*************************** 2. row ***************************
Table: student
Non_unique: 1
Key_name: index_name
Seq_in_index: 1
Column_name: name
Collation: A
Cardinality: NULL
Sub_part: NULL
Packed: NULL
Null:
Index_type: BTREE
Comment:
*************************** 3. row ***************************
Table: student
Non_unique: 1
Key_name: index_dept
Seq_in_index: 1
Column_name: dept
Collation: A
Cardinality: NULL
Sub_part: 8
Packed: NULL
Null: YES
Index_type: BTREE
Comment:
*************************** 4. row ***************************
Table: student
Non_unique: 1
Key_name: ind_name_dept
Seq_in_index: 1
Column_name: name
Collation: A
Cardinality: NULL
Sub_part: NULL
Packed: NULL
Null:
Index_type: BTREE
Comment:
*************************** 5. row ***************************
Table: student
Non_unique: 1
Key_name: ind_name_dept
Seq_in_index: 2
Column_name: dept
Collation: A
Cardinality: NULL
Sub_part: NULL
Packed: NULL
Null: YES
Index_type: BTREE
Comment:
5 rows in set (0.00 sec)可以对多个列的前n个字符创建联合索引，实战演示如下：
[email protected] 06:5144->drop index ind_name_dept on student; 删除索引的另一种方法。
Query OK, 0 rows affected (0.01 sec)
Records: 0 Duplicates: 0 Warnings: 0
[email protected] 06:5158->create index ind_name_dept on student(name(8),dept(10));
Query OK, 0 rows affected (0.00 sec)
Records: 0 Duplicates: 0 Warnings: 0提示：
按条件列查询数据时，联合索引是有前缀生效特性的
Index(a,b,c)仅a,ab,abc三个查询条件可以走索引。b,bc,ac,c不能走索引
5 创建唯一索引（非主键索引）
Create unique index index_age on student(age);
唯一索引是用来约束表的内容的，不能重复。
[email protected] 07:0828->create unique index uni_ind_name on student(name);
Query OK, 0 rows affected (0.00 sec)
Records: 0 Duplicates: 0 Warnings: 0
[email protected] 07:1010->desc student;
+-------+-------------+------+-----+---------+----------------+
| Field | Type| Null | Key | Default | Extra |
+-------+-------------+------+-----+---------+----------------+
| id| int(11) | NO | PRI | NULL| auto_increment |
| name | char(20)| NO | UNI | NULL||
| age | tinyint(2) | NO | | 0 ||
| dept | varchar(16) | YES | MUL | NULL||
+-------+-------------+------+-----+---------+----------------+
4 rows in set (0.00 sec)6 索引表的创建及生效条件
问题1：既然索引可以加快查询速度，那么是不是就要给所有的列创建索引呢？
解答：因为索引你不但占用系统空间，更新数据库时还需要维护索引数据。因此索引是一把双刃剑，并不是越多越好，例如：数十到几百行的小表上无需创建索引，更新频繁，读取少的业务要少建立索引。
问题2：需要在哪些列上创建索引呢？
解答：Select user,host from mysql.user where host= ….索引一定要创建在where后的条件列上，而不是select后的选择数据的列上，另外我们要尽量选择在唯一值多的大表上建立索引。创建索引要和开发商量。
7 创建索引命令集合小结
1.创建索引相关命令集合
创建主键索引
alter table student change id id int primary key auto_increment;删除主键索引
alter table student drop primary key创建普通索引
alter table student add index index_dept(dept(8))根据列的前n个字符创建索引
create index index_dept on student(dept(8))根据多个列创建联合索引
create index ind_name_dept on student(name,dept);根据多个列的前n个字符创建索引
create index ind_name_dept on student(name(8),dept(10));删除普通索引
Alter table student drop index index_dept
drop index ind_name_dept on student创建唯一索引
create unique index uni_ind_name on student(name);基本索引：
a.要在表的列上创建索引。
b.索引会加快查询速度，但是会影响更新的速度，因为要维护索引。
c.索引不是越多越好，要在频繁查询的where后的条件列上创建索引。
d.小表或唯一值极少的列上不建索引，要在大表以及不同内容多的列上创建索引。