# 索引

## 一、创建索引

### 1、创建表的同时创建索引

```mysql
create table 表名(
	列名 数据类型,
	列名 数据类型,
	......
	[unique/fulltext/spatial] + [index/key] (索引名) (列名[长度]) [ASC/DESC]
)
```


#### (1)普通索引

```mysql
create table book(
bookid int not null,
bookname varchar(255) not null,
author varchar(255) not null,
info varchar(255),
comment varchar(255),
year_publication year not null,
index (year_publication)
//index/key(列名)
);
```
```mysql
show create table book;
```

![image-20220407081946290](https://lsky.hhdxw.top/imghub/img/image-20220407081946290.png)

```mysql
explain select * from book where year_publication=1990;
//使用explain查看索引是否正在使用，重点观察结果中的possible keys和key的值，此处都为year_publication，说明执行此查询语句时使用了索引
```

![image-20220407082155889](https://lsky.hhdxw.top/imghub/img/image-20220407082155889.png)

#### (2)唯一索引

```mysql
create table t1(
id int not null,
name char(30) not null,
unique index udid(id)
//unique index 索引名(列名)
//索引名可省略，默认名可show create table 展示
);
```

```mysql
show create table t1;
```

![image-20220407082821945](https://lsky.hhdxw.top/imghub/img/image-20220407082821945.png)

#### (3)组合索引

```mysql
create table t3(
id int not null,
name char(30) not null,
age int not null,
info varchar(255),
index (id,name,age)
);
```

```mysql
explain select * from t3 where id=1 and name='joy';
explain select * from t3 where name='joy';
//查询时，必须遵从最左索引前缀原则
```

![image-20220407084307684](https://lsky.hhdxw.top/imghub/img/image-20220407084307684.png)

#### (4)全文索引 ，只为字符（char，varchar，text）索引

```mysql
create table t4(
id int not null,
name char(30),
age int not null,
info varchar(255),
fulltext index (info)
);
```

```mysql
show create table t4;
```

![image-20220407091100882](https://lsky.hhdxw.top/imghub/img/image-20220407091100882.png)

#### (5)空间索引（ENGIN=MyISAM）

```mysql
create table t5(
g geometry not null,
spatial index(g)
//只能对一些图形等使用
);
```

### 2、在已经存在的表上创建索引

#### (1)使用alter table

```mysql
alter table 表名 add index【索引名】(列名);
//索引名可以省略，默认为列名
alter table book add index(bookname);
//查询索引
show index from 表名;
```

![image-20220407091751882](https://lsky.hhdxw.top/imghub/img/image-20220407091751882.png)

#### (2)使用create index

```mysql
create index [索引名] on 表名(列名);
```



## 二、删除索引

### 1、使用alter table

```mysql
alter table 表名 drop index 索引名; 
alter table book drop index id;
```

![image-20220407093356011](https://lsky.hhdxw.top/imghub/img/image-20220407093356011.png)

### 2、使用drop index

```mysql
drop index 索引名 on 表名; 
drop index bookname on book;
```

![image-20220407093437374](https://lsky.hhdxw.top/imghub/img/image-20220407093437374.png)