MySQL 索引的类型

主键索引（PRIMARY KEY）

# 主键 = 唯一键索引 + 非空 + 只能设置一个字段
# 创建表的时候创建主键索引
mysql> create table test(id int not null auto_increment primary key comment '学号');
Query OK, 0 rows affected (0.04 sec)
mysql> create table test1(id int not null auto_increment,primary key(id));
Query OK, 0 rows affected (0.04 sec)
# 查看索引命令
mysql> show index from test;
mysql> desc test;
# 已经有表时添加主键索引
mysql> alter table student add primary key(id);
mysql> alter table student add primary key pri_id(id);
# 删除主键索引
mysql> alter table student DROP  PRIMARY KEY;

唯一键索引（UNIQUE KEY）

# 唯一键 = 普通索引 + 列中每个值是唯一的
# 创建表的时候创建唯一键索引
mysql> create table test2(id int not null auto_increment unique key comment '学号');
Query OK, 0 rows affected (0.04 sec)
# 已经有表时添加唯一键索引
mysql> alter table student add unique key(name);
mysql> alter table student add unique key uni_key(name);
# 删除唯一键索引
mysql> alter table student drop index name;
mysql> alter table student drop index uni_key;
# 注意：在创建唯一键索引（或主键索引）时，选择的列不能有重复数据
判断一列能否做唯一建索引
1.查询数据总量
mysql> select count(name) from city;
2.去重查看该列数据总量
mysql> select count(distinct name) from city;
# 以上两个值相等则可以设置唯一建索引

如下案例则可以创建 name字段的唯一键索引：

# 1.查看列的总数据量
mysql> select count(name) from country;
+-------------+
| count(name) |
+-------------+
|         239 |
+-------------+
1 row in set (0.00 sec)
# 2.查看去重后数据量
mysql> select count(distinct(name)) from country;
+-----------------------+
| count(distinct(name)) |
+-----------------------+
|                   239 |
+-----------------------+
1 row in set (0.00 sec)
# 3.创建唯一建索引
mysql> alter table country add unique key uni_key(name);

普通索引（INDEX）

# 为了方便查找记录
# 方式一
mysql> alter table city add index inx_name(name);
Query OK, 0 rows affected (0.14 sec)
Records: 0  Duplicates: 0  Warnings: 0
# 方式二
mysql> create index index_District on city(District);
注意，这两种称呼是对建立索引技巧的一种称呼，并非索引的类型。
# 删除索引（方式一，方式二）
mysql> alter table city drop index inx_name;
mysql> drop index index_District on city;

全文索引（FULLTEXT）

mysql> create table txt(id int,bookname varchar(12),wenzhang text,fulltext(wenzhang));
Query OK, 0 rows affected (0.20 sec)
mysql> select * from txt where match(wenzhang) against('查询的内容');
# 实例
mysql> create table text(id int,bookname varchar(12) charset utf8,wenzhang text charset utf8,fulltext(wenzhang));
Query OK, 0 rows affected (0.21 sec)
mysql> insert into text values(1,'红楼梦','上回书说到张飞长坂坡三打白骨精救出宋江');
Query OK, 1 row affected (0.01 sec)
mysql> select * from text;
+------+-----------+-----------------------------------------------------------+
| id   | bookname  | wenzhang                                                  |
+------+-----------+-----------------------------------------------------------+
|    1 | 红楼梦    | 上回书说到张飞长坂坡三打白骨精救出宋江                    |
+------+-----------+-----------------------------------------------------------+
1 row in set (0.00 sec)
mysql> select * from text where match(wenzhang) against('上回书说到张飞长坂坡三打白骨精救出宋江');
+------+-----------+-----------------------------------------------------------+
| id   | bookname  | wenzhang                                                  |
+------+-----------+-----------------------------------------------------------+
|    1 | 红楼梦    | 上回书说到张飞长坂坡三打白骨精救出宋江                    |
+------+-----------+-----------------------------------------------------------+
1 row in set (0.00 sec)
mysql> alter table city drop index wenzhang;
Query OK, 0 rows affected (0.03 sec)
Records: 0  Duplicates: 0  Warnings: 0