150 lines
3.1 KiB
Markdown
150 lines
3.1 KiB
Markdown
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# 索引
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## 一、创建索引
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### 1、创建表的同时创建索引
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```mysql
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create table 表名(
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列名 数据类型,
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列名 数据类型,
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......
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[unique/fulltext/spatial] + [index/key] (索引名) (列名[长度]) [ASC/DESC]
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)
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```
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#### (1)普通索引
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```mysql
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create table book(
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bookid int not null,
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bookname varchar(255) not null,
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author varchar(255) not null,
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info varchar(255),
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comment varchar(255),
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year_publication year not null,
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index (year_publication)
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//index/key(列名)
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);
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```
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```mysql
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show create table book;
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```
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```mysql
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explain select * from book where year_publication=1990;
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//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
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```
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#### (2)唯一索引
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```mysql
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create table t1(
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id int not null,
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name char(30) not null,
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unique index udid(id)
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//unique index 索引名(列名)
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//索引名可省略，默认名可show create table 展示
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);
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```
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```mysql
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show create table t1;
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```
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#### (3)组合索引
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```mysql
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create table t3(
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id int not null,
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name char(30) not null,
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age int not null,
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info varchar(255),
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index (id,name,age)
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);
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```
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```mysql
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explain select * from t3 where id=1 and name='joy';
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explain select * from t3 where name='joy';
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//查询时，必须遵从最左索引前缀原则
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```
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#### (4)全文索引 ，只为字符（char，varchar，text）索引
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```mysql
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create table t4(
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id int not null,
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name char(30),
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age int not null,
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info varchar(255),
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fulltext index (info)
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);
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```
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```mysql
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show create table t4;
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```
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#### (5)空间索引（ENGIN=MyISAM）
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```mysql
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create table t5(
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g geometry not null,
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spatial index(g)
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//只能对一些图形等使用
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);
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```
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### 2、在已经存在的表上创建索引
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#### (1)使用alter table
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```mysql
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alter table 表名 add index【索引名】(列名);
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//索引名可以省略，默认为列名
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alter table book add index(bookname);
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//查询索引
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show index from 表名;
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```
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#### (2)使用create index
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```mysql
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create index [索引名] on 表名(列名);
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```
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## 二、删除索引
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### 1、使用alter table
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```mysql
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alter table 表名 drop index 索引名;
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alter table book drop index id;
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```
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### 2、使用drop index
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```mysql
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drop index 索引名 on 表名;
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drop index bookname on book;
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```
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