【无标题】

库管理

一、建库语法

库：存储表的文件夹

mysql> SHOW CREATE DATABASE tarena \G
*************************** 1. row ***************************
       Database: tarena
Create Database: CREATE DATABASE `tarena` /*!40100 DEFAULT CHARACTER SET latin1 */ /*!80016 DEFAULT ENCRYPTION='N' */
1 row in set (0.00 sec)

create database [if not exists] <数据库名> [[DEFAULT] CHARACTER SET <字符集名>] [[DEFAULT] COLLATE <校对规则名>];

[ ]   表示可选项

if not exists   假设库不存在 避免建库重名报错

character set   指定表使用的字符集（存储字符串的方式）

collate   指定校对规则（比较字符串的方式）

二、常用命令

查看已有的库        show databases;

切换库        use 库名;

删除库        drop database 库名;        （drop database if exists 库名; ）

创建库        create database 库名;

所在库        select database();

三、库名的命名规则

仅可以使用数字、字母、下划线，不能纯数字

区分大小写

具有唯一性

不可使用MySQL命令或特殊字符

创建表

就是创建存储数据的表头名

一、语法

create    table   库名.表名(

表头名   数据类型,

表头名   数据类型,

......

);

二、常用命令

删除表               drop table 库名.表名;

删除表记录        delete from 库名.表名;

修改表记录        update 库名.表名 set 表头名="表头新值" where;

插入表记录        insert into 表名 values(表头的值,...,...),( )...;                                 

                          insert into 库.表(表头名,...) values(表头的值,...);                  

三、修改表结构       

alter table 库名.表名 执行动作;

1、添加表头 

添加新字段，一起添加多个字段使用‘  ，’分隔 add 命令（first，after）

alter table 库名.表名 add 表头名 数据类型;

mysql> alter table user add email char(30);
mysql> select * from user limit 1;
+----+--------+----------+------+------+---------+----------------+---------------+-------+
| id | name   | password | uid  | gid  | comment | homedir        | shell         | email |
+----+--------+----------+------+------+---------+----------------+---------------+-------+
|  1 | root   | x        |    0 |    0 | root    | /root          | /bin/bash     | NULL  |

mysql> alter table user add sex char(3),add banji char(7);
mysql> select * from user limit 1;
+----+--------+----------+------+------+---------+----------------+---------------+-------+------+-------+
| id | name   | password | uid  | gid  | comment | homedir        | shell         | email | sex  | banji |
+----+--------+----------+------+------+---------+----------------+---------------+-------+------+-------+
|  1 | root   | x        |    0 |    0 | root    | /root          | /bin/bash     | NULL  | NULL | NULL  |

mysql> alter table user add school char(10) first;
mysql> select * from user limit 1;
+--------+----+--------+----------+------+------+---------+----------------+---------------+-------+------+-------+
| school | id | name   | password | uid  | gid  | comment | homedir        | shell         | email | sex  | banji |
+--------+----+--------+----------+------+------+---------+----------------+---------------+-------+------+-------+
| NULL   |  1 | root   | x        |    0 |    0 | root    | /root          | /bin/bash     | NULL  | NULL | NULL  |

mysql> alter table user add shenfenzheng char(18) after name;
mysql> select * from user limit 1;
+----+------+--------------+----------+---------+---------+-----------+-------+------+-------+
| id | name | shenfenzheng | password | comment | homedir | shell     | email | sex  | banji |
+----+------+--------------+----------+---------+---------+-----------+-------+------+-------+
|  1 | root | NULL         | x        | root    | /root   | /bin/bash | NULL  | NULL | NULL  |
+----+------+--------------+----------+---------+---------+-----------+-------+------+-------+

2、删除表头 

删除字段，删除多个字段使用‘  ，’分隔drop命令

alter table 库名.表名 drop 表头名;

mysql> alter table user drop school;
mysql> select * from user limit 1;
+----+------+----------+------+------+---------+---------+-----------+-------+------+-------+
| id | name | password | uid  | gid  | comment | homedir | shell     | email | sex  | banji |
+----+------+----------+------+------+---------+---------+-----------+-------+------+-------+
|  1 | root | x        |    0 |    0 | root    | /root   | /bin/bash | NULL  | NULL | NULL  |
+----+------+----------+------+------+---------+---------+-----------+-------+------+-------+

mysql> alter table user drop uid,drop gid;
mysql> select * from user limit 1;
+----+------+----------+---------+---------+-----------+-------+------+-------+
| id | name | password | comment | homedir | shell     | email | sex  | banji |
+----+------+----------+---------+---------+-----------+-------+------+-------+
|  1 | root | x        | root    | /root   | /bin/bash | NULL  | NULL | NULL  |
+----+------+----------+---------+---------+-----------+-------+------+-------+

3、修改表名 

alter table 库名.表名 rename 库名.新表名;

mysql> show tables;
+------------------+
| Tables_in_tarena |
+------------------+
......
| wage_grade       |
+------------------+
5 rows in set (0.00 sec)

mysql> alter table wage_grade rename 工资表级表;

mysql> show tables;
+------------------+
| Tables_in_tarena |
+------------------+
......
| 工资表级表       |
+------------------+
5 rows in set (0.00 sec)

4、修改表头名

修改字段名，也可以同时修改字段类型

alter table 库名.表名 change 原表头名 新表头名 数据类型;

mysql> desc 工资表级表;
+-------+---------+------+-----+---------+-------+
| Field | Type    | Null | Key | Default | Extra |
+-------+---------+------+-----+---------+-------+
| id    | int     | YES  |     | NULL    |       |
| grade | char(1) | YES  |     | NULL    |       |
| low   | int     | YES  |     | NULL    |       |
| high  | int     | YES  |     | NULL    |       |
+-------+---------+------+-----+---------+-------+
4 rows in set (0.00 sec)

mysql> alter table 工资表级表 change low 最低值 int;

mysql> desc 工资表级表;
+-----------+---------+------+-----+---------+-------+
| Field     | Type    | Null | Key | Default | Extra |
+-----------+---------+------+-----+---------+-------+
| id        | int     | YES  |     | NULL    |       |
| grade     | char(1) | YES  |     | NULL    |       |
| 最低值    | int     | YES  |     | NULL    |       |
| high      | int     | YES  |     | NULL    |       |
+-----------+---------+------+-----+---------+-------+
4 rows in set (0.00 sec)

5、修改表头位置

修改字段类型，也可修改字段的位置

alter table 库名.表名 modify 表头名 数据类型 after 表头名;

mysql> alter table tarena.工资表级表 modify high int after grade;

mysql> select * from tarena.工资表级表;
+------+-------+---------+-----------+
| id   | grade | high    | 最低值    |
+------+-------+---------+-----------+
|    1 | A     |    8000 |      5000 |
|    2 | B     |   10000 |      8001 |
|    3 | C     |   15000 |     10001 |
|    4 | D     |   20000 |     15001 |
|    5 | E     | 1000000 |     20001 |
+------+-------+---------+-----------+
5 rows in set (0.00 sec)

修改数据类型 

alter table 库名.表名 modify 表头名 新数据类型;         

如果表头下没有存储数据，类型随便改；反之 修改的新类型不能与已经存储的数据冲突，冲突的话不许修改

mysql> desc user;
+--------------+-------------+------+-----+---------+----------------+
| Field        | Type        | Null | Key | Default | Extra          |
+--------------+-------------+------+-----+---------+----------------+
......
| name         | char(20)    | YES  |     | NULL    |                |
......
+--------------+-------------+------+-----+---------+----------------+
10 rows in set (0.01 sec)

mysql> alter table user modify name varchar(3);
ERROR 1406 (22001): Data too long for column 'name' at row 1
mysql> alter table user modify name varchar(12);
ERROR 1406 (22001): Data too long for column 'name' at row 14

mysql> alter table user modify name varchar(15);
Query OK, 27 rows affected (0.71 sec)
Records: 27  Duplicates: 0  Warnings: 0



mysql> desc user;    # 此时 shenfenzheng 表头值为空
+--------------+-------------+------+-----+---------+----------------+
| Field        | Type        | Null | Key | Default | Extra          |
+--------------+-------------+------+-----+---------+----------------+
......
| shenfenzheng | char(18)    | YES  |     | NULL    |                |
......
+--------------+-------------+------+-----+---------+----------------+
10 rows in set (0.00 sec)

mysql> alter table user modify shenfenzheng char(6);
Query OK, 27 rows affected (1.02 sec)
Records: 27  Duplicates: 0  Warnings: 0

复制表

和系统的cp命令功能一样

表由表头和数据组成

一、仅仅备份表头

create table 备份到哪个库.备份到哪个表 like 库名.原表;

mysql> create database db1;
mysql> create table db1.wage_garde like tarena.工资表级表;
mysql> use db1;
mysql> show tables;
+---------------+
| Tables_in_db1 |
+---------------+
| wage_garde    |
+---------------+
1 row in set (0.00 sec)

mysql> desc wage_garde;
+-----------+---------+------+-----+---------+-------+
| Field     | Type    | Null | Key | Default | Extra |
+-----------+---------+------+-----+---------+-------+
| id        | int     | YES  |     | NULL    |       |
| grade     | char(1) | YES  |     | NULL    |       |
| 最低值    | int     | YES  |     | NULL    |       |
| high      | int     | YES  |     | NULL    |       |
+-----------+---------+------+-----+---------+-------+
4 rows in set (0.00 sec)

mysql> select * from  wage_garde;
Empty set (0.00 sec)

二、备份表头+数据

mysql> create table db1.gzdj2 select * from tarena.工资表级表;
mysql> show tables
    -> ;
+---------------+
| Tables_in_db1 |
+---------------+
| gzdj2         |
| wage_garde    |
+---------------+
2 rows in set (0.01 sec)

mysql> desc gzdj2;
+-----------+---------+------+-----+---------+-------+
| Field     | Type    | Null | Key | Default | Extra |
+-----------+---------+------+-----+---------+-------+
| id        | int     | YES  |     | NULL    |       |
| grade     | char(1) | YES  |     | NULL    |       |
| 最低值    | int     | YES  |     | NULL    |       |
| high      | int     | YES  |     | NULL    |       |
+-----------+---------+------+-----+---------+-------+
4 rows in set (0.00 sec)

mysql> select * from gzdj2;
+------+-------+-----------+---------+
| id   | grade | 最低值    | high    |
+------+-------+-----------+---------+
|    1 | A     |      5000 |    8000 |
|    2 | B     |      8001 |   10000 |
|    3 | C     |     10001 |   15000 |
|    4 | D     |     15001 |   20000 |
|    5 | E     |     20001 | 1000000 |
+------+-------+-----------+---------+
5 rows in set (0.00 sec)

# 仅备份符合条件的数据
mysql> select employee_id,basic from tarena.salary where year(date)=2018 and month(date)=7 and basic > 15000;
+-------------+-------+
| employee_id | basic |
+-------------+-------+
......
62 rows in set (0.00 sec)

mysql> create table db1.salary7 select employee_id,basic from tarena.salary where year(date)=2018 and month(date)=7 and basic > 15000;

mysql> select * from salary7;
+-------------+-------+
| employee_id | basic |
+-------------+-------+
......
62 rows in set (0.00 sec)

数据类型

一、查看已有表里表头存储数据使用的类型

二、了解MySQL服务支持的数据类型

三、掌握常用数据类型的使用

每种数据类型使用固定的命令表示且由固定的存储数据范围

1、数值类型

存储数字的表头使用数值类型

1）整型类型

只存储数值的整数部分（遇小数四舍五入）

2）浮点类型

既存整数又能存储有小数点的数

mysql> create table db1.t12(
    -> 人物 varchar(10),
    -> 等级 tinyint unsigned,
    -> 点劵 float
    -> );
Query OK, 0 rows affected (0.33 sec)
mysql> insert into db1.t12 values("法师",-1,-98);
ERROR 1264 (22003): Out of range value for column '等级' at row 1
mysql> insert into db1.t12 values("法师",0,-98);
Query OK, 1 row affected (0.04 sec)
mysql> insert into db1.t12 values("战士",256,12.25);
ERROR 1264 (22003): Out of range value for column '等级' at row 1
mysql> insert into db1.t12 values("战士",25,12.25);
Query OK, 1 row affected (0.10 sec)
mysql> select * from db1.t12;
+--------+--------+--------+
| 人物   | 等级   | 点劵   |
+--------+--------+--------+
| 法师   |      0 |    -98 |
| 战士   |     25 |  12.25 |
+--------+--------+--------+
2 rows in set (0.00 sec)
mysql> insert into db1.t12 values("士兵",19.25,12.25);
Query OK, 1 row affected (0.05 sec)

mysql> select * from db1.t12;
+--------+--------+--------+
| 人物   | 等级   | 点劵   |
+--------+--------+--------+
| 法师   |      0 |    -98 |
| 战士   |     25 |  12.25 |
| 士兵   |     19 |  12.25 |
+--------+--------+--------+
3 rows in set (0.00 sec)

mysql> insert into db1.t12 values("士兵2",19.56,12.25);
Query OK, 1 row affected (0.05 sec)

mysql> select * from db1.t12;
+---------+--------+--------+
| 人物    | 等级   | 点劵   |
+---------+--------+--------+
| 法师    |      0 |    -98 |
| 战士    |     25 |  12.25 |
| 士兵    |     19 |  12.25 |
| 士兵2   |     20 |  12.25 |
+---------+--------+--------+
4 rows in set (0.00 sec)

2、字符类型

1）char

定长字符类型   最多255个字符

不够指定字符个数时在右边用空格补全字符个数，超出时无法写入数据

2）varchar   

变长字符类型   最多65532个字符

按数据实际大小分配存储空间，字符个数超出时无法写入数据

mysql> create table db1.t11(
    -> 姓名 char(4),
    -> 住址 varchar(20)
    -> );
Query OK, 0 rows affected (0.58 sec)

mysql> insert into db1.t11 values("牛牛","天桥下");
Query OK, 1 row affected (0.05 sec)

mysql> insert into db1.t11 values("牛牛爱锤子","天桥下");
ERROR 1406 (22001): Data too long for column '姓名' at row 1

mysql> select * from db1.t11;
+--------+-----------+
| 姓名   | 住址      |
+--------+-----------+
| 牛牛   | 天桥下    |
+--------+-----------+
1 row in set (0.00 sec)

mysql> desc db1.t11;
+--------+-------------+------+-----+---------+-------+
| Field  | Type        | Null | Key | Default | Extra |
+--------+-------------+------+-----+---------+-------+
| 姓名   | char(4)     | YES  |     | NULL    |       |
| 住址   | varchar(20) | YES  |     | NULL    |       |
+--------+-------------+------+-----+---------+-------+
2 rows in set (0.00 sec)

3、枚举类型

性别、爱好

表头的值必须在定义的范围内选择

单选 enum(值列表)

多选 set(值列表)

mysql> create table db1.t13( 姓名 char(10),性别 enum("男","女"),爱好 set("吃","睡","玩","钱") );
Query OK, 0 rows affected (0.56 sec)

mysql> desc db1.t13;
+--------+------------------------------+------+-----+---------+-------+
| Field  | Type                         | Null | Key | Default | Extra |
+--------+------------------------------+------+-----+---------+-------+
| 姓名   | char(10)                     | YES  |     | NULL    |       |
| 性别   | enum('男','女')              | YES  |     | NULL    |       |
| 爱好   | set('吃','睡','玩','钱')     | YES  |     | NULL    |       |
+--------+------------------------------+------+-----+---------+-------+
3 rows in set (0.01 sec)

mysql> insert into db1.t13 values("a","男","吃,睡");
Query OK, 1 row affected (0.06 sec)

mysql> insert into db1.t13 values("b","公公","it,学习");
ERROR 1265 (01000): Data truncated for column '性别' at row 1
mysql> insert into db1.t13 values("b","女","it,学习");
ERROR 1265 (01000): Data truncated for column '爱好' at row 1
mysql> insert into db1.t13 values("b","女","玩");
Query OK, 1 row affected (0.05 sec)

mysql> select * from db1.t13;
+--------+--------+---------+
| 姓名   | 性别   | 爱好    |
+--------+--------+---------+
| a      | 男     | 吃,睡   |
| b      | 女     | 玩      |
+--------+--------+---------+
2 rows in set (0.00 sec)

4、日期时间类型

类型	名称	范围	赋值格式	20(0-69) 19(70-99)
year	年	1901～2155	YYYY
date	日期	0001-01-01～9999-12-31	YYYYMMDD
time	时间	01:00:00～23:59:59	HHMMSS
datetime	日期时间	1000-01-01 00:00:00～9999-12-31 23:59:59	YYYYMMDDHHMMSS
timestamp		1970-01-01 00:00:00～2038-01-19 00:00:00

mysql> create table db1.byb( 姓名 char(4), 班级 char(7), 出生年份 year, 生日 date, 上课时间 time, 双选会时间 datetime, 结业聚餐 timestamp );
Query OK, 0 rows affected (0.82 sec)

mysql> desc byb;
+-----------------+-----------+------+-----+---------+-------+
| Field           | Type      | Null | Key | Default | Extra |
+-----------------+-----------+------+-----+---------+-------+
| 姓名            | char(4)   | YES  |     | NULL    |       |
| 班级            | char(7)   | YES  |     | NULL    |       |
| 出生年份        | year      | YES  |     | NULL    |       |
| 生日            | date      | YES  |     | NULL    |       |
| 上课时间        | time      | YES  |     | NULL    |       |
| 双选会时间      | datetime  | YES  |     | NULL    |       |
| 结业聚餐        | timestamp | YES  |     | NULL    |       |
+-----------------+-----------+------+-----+---------+-------+
7 rows in set (0.00 sec)

mysql> insert into byb values("牛牛","nsd2310",1990,19901120,090000,20240228083000,20240215203000);
Query OK, 1 row affected (0.06 sec)

mysql> select * from byb;
+--------+---------+--------------+------------+--------------+---------------------+---------------------+
| 姓名   | 班级    | 出生年份     | 生日       | 上课时间     | 双选会时间          | 结业聚餐            |
+--------+---------+--------------+------------+--------------+---------------------+---------------------+
| 牛牛   | nsd2310 |         1990 | 1990-11-20 | 09:00:00     | 2024-02-28 08:30:00 | 2024-02-15 20:30:00 |
+--------+---------+--------------+------------+--------------+---------------------+---------------------+
1 row in set (0.00 sec)

mysql> insert into byb(姓名,出生年份) values("a",69),("b",70),("c",81);
Query OK, 3 rows affected (0.06 sec)
Records: 3  Duplicates: 0  Warnings: 0

mysql> select * from byb;
+--------+---------+--------------+------------+--------------+---------------------+---------------------+
| 姓名   | 班级    | 出生年份     | 生日       | 上课时间     | 双选会时间          | 结业聚餐            |
+--------+---------+--------------+------------+--------------+---------------------+---------------------+
| 牛牛   | nsd2310 |         1990 | 1990-11-20 | 09:00:00     | 2024-02-28 08:30:00 | 2024-02-15 20:30:00 |
| a      | NULL    |         2069 | NULL       | NULL         | NULL                | NULL                |
| b      | NULL    |         1970 | NULL       | NULL         | NULL                | NULL                |
| c      | NULL    |         1981 | NULL       | NULL         | NULL                | NULL                |
+--------+---------+--------------+------------+--------------+---------------------+---------------------+
4 rows in set (0.00 sec)

 数据批量处理

一、检索目录

存储数据的文件必须存放在检索目录里

1、查看默认的检索目录

1）查看所有的配置项

show variables;

2）查看和某类配置相关

show variables like "%关键字%";

比如 show variables like "%file%";

        show variables like "%password%";

3）查看指定配置项的值

show variables like "secure_file_priv";

mysql> show variables like "secure_file_priv";
+------------------+-----------------------+
| Variable_name    | Value                 |
+------------------+-----------------------+
| secure_file_priv | /var/lib/mysql-files/ |
+------------------+-----------------------+
1 row in set (0.01 sec)

[root@mysql50 ~]# ll -d /var/lib/mysql-files/
drwxr-x--- 2 mysql mysql 6 Sep 22  2021 /var/lib/mysql-files/
[root@mysql50 ~]# ls /var/lib/mysql-files/    #空目录

2、自定义检索目录

[root@mysql50 ~]# mkdir /myload
[root@mysql50 ~]# chown mysql /myload/
[root@mysql50 ~]# getenforce    # 模式不能为 enforcing
Disabled
[root@mysql50 ~]# vim /etc/my.cnf.d/mysql-server.cnf 
[mysqld]
......
secure_file_priv=/myload    # 添加此行
[root@mysql50 ~]# systemctl restart mysqld.service 

# 查看验证
mysql> show variables like "secure_file_priv";
+------------------+----------+
| Variable_name    | Value    |
+------------------+----------+
| secure_file_priv | /myload/ |
+------------------+----------+
1 row in set (0.01 sec)

mysql用户对 检索目录的父目录 必须有 r-x 权限 

一、数据导入

把系统文件的内容，存储到数据库的表里

load data infile "/myload/文件名" into table 库.表 field terminated by "文件里列的间隔符号" lines terminated by "\n";

步骤：

创建数据库

mysql> create database db3;

创建表（根据文件的内容创建表头和数据类型）

mysql> create table db3.user(
    -> name varchar(20),
    -> password char(1),
    -> uid int,gid int,
    -> comment varchar(200),
    -> homedir varchar(30),
    -> shell varchar(30)
    -> );

mysql> use db3;

mysql> show tables;
+---------------+
| Tables_in_db3 |
+---------------+
| user          |
+---------------+
1 row in set (0.00 sec)
mysql> desc user;
+----------+--------------+------+-----+---------+-------+
| Field    | Type         | Null | Key | Default | Extra |
+----------+--------------+------+-----+---------+-------+
| name     | varchar(20)  | YES  |     | NULL    |       |
| password | char(1)      | YES  |     | NULL    |       |
| uid      | int          | YES  |     | NULL    |       |
| gid      | int          | YES  |     | NULL    |       |
| comment  | varchar(200) | YES  |     | NULL    |       |
| homedir  | varchar(30)  | YES  |     | NULL    |       |
| shell    | varchar(30)  | YES  |     | NULL    |       |
+----------+--------------+------+-----+---------+-------+
7 rows in set (0.00 sec)

mysql> select * from user;
Empty set (0.00 sec)

把文件拷贝到检索目录里

mysql> system cp /etc/passwd /myload
mysql> system ls /myload
passwd

指定导入数据的命令

mysql> system wc -l /myload/passwd
24 /myload/passwd
mysql> load data infile "/myload/passwd" into table db3.user fields terminated by ":" lines terminated by "\n";

查看表里的行

mysql> select count(*) from user;
+----------+
| count(*) |
+----------+
|       24 |
+----------+
1 row in set (0.00 sec)

二、数据导出

把表里的行，存放到系统文件里

select查询命令 into outfile "/检索目录/文件名";

mysql> select * from tarena.salary where year(date)=2018 and employee_id=6;
mysql> select * from tarena.salary where year(date)=2018 and employee_id=6 into outfile "/myload/salary.txt";
mysql> system vim /myload/salary.txt

改变列和列的间隔符号

select查询命令 into outfile "/检索目录/文件名" fields terminated "符号";

mysql> select employee_id,basic,bonus from tarena.salary where year(date)=2018 and employee_id=6 into outfile "/myload/salary_2.txt" fields terminated by ":";

表头基本约束

约束设置在表头上 作用是如何给表头赋值

一、基本约束

1、不允许赋null值

2、值必须唯一（唯一索引）

3、设置默认值（不给表头赋值时，使用默认值赋值；建表时如果没有定义默认值，mysql服务分配的默认值是null）

二、查看表头约束

desc 库.表;

Null | Key | Default | Extra

 空      键值  默认值  额外设置

建表时设置约束

create table db3.t1( name char(3) not null,age int default 19,class char(7) default "nsd2310";

create table db3.t3( number int,name char(3) not null,age int default 19,class char(7) default "nsd2310" unique(number);

mysql> create table db3.t6(
    -> number int,
    -> name char(3),
    -> age int,
    -> class char(7)
    -> );
Query OK, 0 rows affected (0.32 sec)

mysql> desc db3.t6;
+--------+---------+------+-----+---------+-------+
| Field  | Type    | Null | Key | Default | Extra |
+--------+---------+------+-----+---------+-------+
| number | int     | YES  |     | NULL    |       |
| name   | char(3) | YES  |     | NULL    |       |
| age    | int     | YES  |     | NULL    |       |
| class  | char(7) | YES  |     | NULL    |       |
+--------+---------+------+-----+---------+-------+
4 rows in set (0.00 sec)

mysql> alter table db3.t6 modify name char(3) not null;
Query OK, 0 rows affected (0.55 sec)
Records: 0  Duplicates: 0  Warnings: 0

mysql> alter table db3.t6 modify age int not null default 19;;
Query OK, 0 rows affected (0.81 sec)
Records: 0  Duplicates: 0  Warnings: 0

ERROR: 
No query specified

mysql> create unique index a on db3.t6(number);    # 设置唯一索引
Query OK, 0 rows affected (0.18 sec)
Records: 0  Duplicates: 0  Warnings: 0

mysql> desc db3.t6;
+--------+---------+------+-----+---------+-------+
| Field  | Type    | Null | Key | Default | Extra |
+--------+---------+------+-----+---------+-------+
| number | int     | YES  | UNI | NULL    |       |
| name   | char(3) | NO   |     | NULL    |       |
| age    | int     | NO   |     | 19      |       |
| class  | char(7) | YES  |     | NULL    |       |
+--------+---------+------+-----+---------+-------+
4 rows in set (0.00 sec)

mysql> show index from db3.t6\G    # 查看索引
*************************** 1. row ***************************
        Table: t6
   Non_unique: 0
     Key_name: a
 Seq_in_index: 1
  Column_name: number
    Collation: A
  Cardinality: 0
     Sub_part: NULL
       Packed: NULL
         Null: YES
   Index_type: BTREE
      Comment: 
Index_comment: 
      Visible: YES
   Expression: NULL
1 row in set (0.05 sec)

mysql> drop index a on db3.t6;    # 删除索引
Query OK, 0 rows affected (0.19 sec)
Records: 0  Duplicates: 0  Warnings: 0

mysql> show index from db3.t6\G
Empty set (0.00 sec)