mysql学习第三天

mysql管理表和索引

数据库

创建库

获取创建库的帮助

mysql> help create database;    //获取帮助如何创建数据库
Name: 'CREATE DATABASE'
Description:
Syntax:
CREATE {DATABASE | SCHEMA} [IF NOT EXISTS] db_name
    [create_specification(额外的选项)] ...
​
create_specification:
    [DEFAULT] CHARACTER SET [=] charset_name
  | [DEFAULT] COLLATE [=] collation_name
​
CREATE DATABASE creates a database with the given name. To use this
statement, you need the CREATE privilege for the database. CREATE
SCHEMA is a synonym for CREATE DATABASE.
​
URL: http://dev.mysql.com/doc/refman/5.5/en/create-database.html
​

创建一个新库的过程：

mysql> CREATE DATABASE IF NOT EXISTS students CHARACTER SET 'gbk' COLLATE 'gbk_chinese_ci';
Query OK, 1 row affected (0.00 sec)
​

在数据目录里面会生成db.opt文件：

[root@mysql students]# pwd
/mysql/students
[root@mysql students]# ls
db.opt
[root@mysql students]# file db.opt 
db.opt: ASCII text  /文件类型为ascii文件
[root@mysql students]# cat db.opt 
default-character-set=gbk   //保存内容为字符集和排序规则
default-collation=gbk_chinese_ci

修改库

修改用ALTER命令

mysql> HELP ALTER DATABASE  //使用该命令查看如何修改数据库
Name: 'ALTER DATABASE'
Description:
Syntax:
ALTER {DATABASE | SCHEMA} [db_name]
    alter_specification ...
ALTER {DATABASE | SCHEMA} db_name
    UPGRADE DATA DIRECTORY NAME
​
alter_specification:
    [DEFAULT] CHARACTER SET [=] charset_name
  | [DEFAULT] COLLATE [=] collation_name
//能修改数据库的部分：字符集和排序规则，还可以升级数据字典名称

修改：

mysql> ALTER DATABASE students CHARACTER SET 'gbk' COLLATE 'gbk_bin';
Query OK, 1 row affected (0.00 sec)

删除库

获取删除库的帮助

mysql> HELP DROP DATABASE;
Name: 'DROP DATABASE'
Description:
Syntax:
DROP {DATABASE | SCHEMA} [IF EXISTS] db_name
​

删除：

mysql> DROP SCHEMA students;
Query OK, 0 rows affected (0.00 sec)
​
mysql> SHOW DATABASES;
+--------------------+
| Database           |
+--------------------+
| information_schema |
| fsx                |
| mysql              |
| performance_schema |
| test               |
+--------------------+
5 rows in set (0.00 sec)
​

在数据目录里面查看：

[root@mysql mysql]# ls
fsx      ib_logfile0  mysql             mysql-bin.index  mysql.pid           test
ibdata1  ib_logfile1  mysql-bin.000001  mysql.err        performance_schema
//已经没有了students数据库

数据库改名：

可以关闭服务器，在数据目录对文件重命名，即可。

或者新建数据库，数据迁移过去。一般情况不建议数据库重命名。

表

学习表之前，先使用该表

mysql>  use students
Database changed
​

创建表

创建表一般情况有三种方法，使用HELP可以查看具体使用方法

mysql> HELP CREATE TABLE;   //三种方式，这里的帮助信息不全，具体执行后查看
Name: 'CREATE TABLE'
Description:
Syntax:
CREATE [TEMPORARY] TABLE [IF NOT EXISTS] tbl_name
    (create_definition,...)
    [table_options]
    [partition_options]
​
CREATE [TEMPORARY] TABLE [IF NOT EXISTS] tbl_name
    [(create_definition,...)]
    [table_options]
    [partition_options]
    [IGNORE | REPLACE]
    [AS] query_expression
​
CREATE [TEMPORARY] TABLE [IF NOT EXISTS] tbl_name
    { LIKE old_tbl_name | (LIKE old_tbl_name) }
​

第一种：直接定义一张空表

CREATE TABLE IF NOT EXISTS table_name (col_name col_defination(最核心的是字段类型)(字段名称字段定义),constraint(约束)) ENGINE = engine_name(存储引擎) MAX_ROWS 数字(最多存储多少行)

第二种：从其它表中查询数据，并以之创建新表

第三种：仿照其他表，以之为模板，创建一个空表

关于PRIMARY KEY和UNIQUE KEY，如果是单字段，可以在定义字段信息是，后面添加。如果是多字段组合，可以最后单独定义PRIMARY KEY()，括号里面是字段名称。

**创建：**

方法一：

mysql> CREATE TABLE IF NOT EXISTS class_1(id INT UNSIGNED NOT NULL AUTO_INCREMENT PRIMARY KEY ,name char(20) NOT NULL,age TINYINT NOT NULL);//创建表
Query OK, 0 rows affected (0.15 sec)
​
mysql> SHOW TABLES;
+--------------------+
| Tables_in_students |
+--------------------+
| class_1            |
+--------------------+
1 row in set (0.00 sec)
​
mysql> CREATE TABLE IF NOT EXISTS class_2(id INT UNSIGNED NOT NULL AUTO_INCREMENT ,name char(20) NOT NULL,age TINYINT NOT NULL,PRIMARY KEY(id));//如果主键是字段之和，就必须单独列出来
Query OK, 0 rows affected (0.15 sec)
​
mysql> SHOW TABLES;
+--------------------+
| Tables_in_students |
+--------------------+
| class_1            |
| class_2            |
+--------------------+
2 rows in set (0.00 sec)
​
mysql> CREATE TABLE IF NOT EXISTS class_3(id INT UNSIGNED NOT NULL AUTO_INCREMENT ,name char(20) NOT NULL,age TINYINT NOT NULL,PRIMARY KEY(id),UNIQUE(name,age));//增加唯一键
Query OK, 0 rows affected (0.25 sec)
​
mysql> SHOW TABLES;
+--------------------+
| Tables_in_students |
+--------------------+
| class_1            |
| class_2            |
| class_3            |
+--------------------+
3 rows in set (0.00 sec)
​
mysql> CREATE TABLE IF NOT EXISTS class_4(id INT UNSIGNED NOT NULL AUTO_INCREMENT ,name char(20) NOT NULL,age TINYINT NOT NULL,PRIMARY KEY(id),UNIQUE(name,age),INDEX(age));//创建索引(age),索引允许值相同，索引结构都为B+Tree
Query OK, 0 rows affected (0.40 sec)
​
mysql> SHOW TABLES;
+--------------------+
| Tables_in_students |
+--------------------+
| class_1            |
| class_2            |
| class_3            |
| class_4            |
+--------------------+
4 rows in set (0.00 sec)
mysql> CREATE TABLE courses(cid TINYINT UNSIGNED NOT NULL AUTO_INCREMENT PRIMARY KEY,course VARCHAR(50) NOT NULL) ENGINE=MyISAM;    //指定存储引擎为MyISAM
Query OK, 0 rows affected (0.35 sec)
​
mysql> show tables;
+--------------------+
| Tables_in_students |
+--------------------+
| class_1            |
| class_2            |
| class_3            |
| class_4            |
| courses            |
+--------------------+
5 rows in set (0.00 sec)
​
//键也称作约束，可用作索引，属于特殊索引(有特殊限定)，都是B+Tree结构

查看表class_1信息：

mysql> show table status like '%class_1' \G
*************************** 1. row ***************************
           Name: class_1
         Engine: InnoDB //存储引擎
        Version: 10     //版本号
     Row_format: Compact    //行格式(此处是压缩格式)
           Rows: 0
 Avg_row_length: 0
    Data_length: 16384  //数据长度(大小)
Max_data_length: 0
   Index_length: 0
      Data_free: 10485760
 Auto_increment: 1
    Create_time: 2018-05-10 18:36:26    //创建时间
    Update_time: NULL   //上次更新时间
     Check_time: NULL
      Collation: gbk_chinese_ci //排序规则，继承数据库
       Checksum: NULL
 Create_options: 
        Comment: 
1 row in set (0.00 sec)
//不同的存储引擎，表信息会有不同，上面的是InnoDB，下面是MyISAM
mysql> SHOW TABLE STATUS LIKE '%courses'\G
*************************** 1. row ***************************
           Name: courses    
         Engine: MyISAM //存储引擎为MyISAM
        Version: 10
     Row_format: Dynamic    //行格式为动态
           Rows: 0
 Avg_row_length: 0
    Data_length: 0
Max_data_length: 281474976710655    //最大数据长度
   Index_length: 1024
      Data_free: 0
 Auto_increment: 1
    Create_time: 2018-05-10 18:56:05
    Update_time: 2018-05-10 18:56:05
     Check_time: NULL
      Collation: gbk_chinese_ci
       Checksum: NULL
 Create_options: 
        Comment: 
1 row in set (0.00 sec)
​

表中插入数据

mysql> INSERT INTO courses (course) values ('数学'),('语文'),('英语');
Query OK, 3 rows affected (0.00 sec)
Records: 3  Duplicates: 0  Warnings: 0
​
mysql> select * from courses;
+-----+--------+
| cid | course |
+-----+--------+
|   1 | 数学   |
|   2 | 语文   |
|   3 | 英语   |
+-----+--------+
3 rows in set (0.00 sec)
//注意cid是自动生成的，因为有AUTO_CREMENT函数
mysql> SHOW INDEXES FROM courses \G //显示表上的索引信息
*************************** 1. row ***************************
        Table: courses  //表名
   Non_unique: 0    //是不是非唯一键，这里是唯一键
     Key_name: PRIMARY  //键名称，是主键
 Seq_in_index: 1    //第几个索引
  Column_name: cid  //创建到cid字段
    Collation: A    //排序规则
  Cardinality: 3    
     Sub_part: NULL 
       Packed: NULL
         Null: 
   Index_type: BTREE    //索引类型B-Tree类型
      Comment: 
Index_comment: 
1 row in set (0.00 sec)

方法二：

这里看到courses表，就仿照它创建一个表：

mysql> CREATE TABLE testcourses SELECT * FROM courses WHERE cid <= 2;
Query OK, 2 rows affected (0.16 sec)
Records: 2  Duplicates: 0  Warnings: 0
​
mysql> SHOW TABLES;
+--------------------+
| Tables_in_students |
+--------------------+
| class_1            |
| class_2            |
| class_3            |
| courses            |
| testcourses        |
+--------------------+
5 rows in set (0.00 sec)
mysql> SELECT * FROM testcourses;
+-----+--------+
| cid | course |
+-----+--------+
|   1 | 数学   |
|   2 | 语文   |
+-----+--------+
2 rows in set (0.00 sec)
mysql> DESC courses;    //显示表结构
+--------+---------------------+------+-----+---------+----------------+
| Field  | Type                | Null | Key | Default | Extra          |
+--------+---------------------+------+-----+---------+----------------+
| cid    | tinyint(3) unsigned | NO   | PRI | NULL    | auto_increment |
| course | varchar(50)         | NO   |     | NULL    |                |
+--------+---------------------+------+-----+---------+----------------+
2 rows in set (0.00 sec)
mysql> DESC testcourses;
+--------+---------------------+------+-----+---------+-------+
| Field  | Type                | Null | Key | Default | Extra |
+--------+---------------------+------+-----+---------+-------+
| cid    | tinyint(3) unsigned | NO   |     | 0       |       |
| course | varchar(50)         | NO   |     | NULL    |       |
+--------+---------------------+------+-----+---------+-------+
2 rows in set (0.00 sec)
//注意：尽管是模仿创建的表，但是很多表属性定义等会不存在了

方法三：

mysql> CREATE TABLE test LIKE courses;  //第三种建立表的方式
Query OK, 0 rows affected (0.11 sec)
​
mysql> SHOW TABLES;
+--------------------+
| Tables_in_students |
+--------------------+
| class_1            |
| class_2            |
| class_3            |
| courses            |
| test               |
| testcourses        |
+--------------------+
6 rows in set (0.00 sec)
​
mysql> DESC test;
+--------+---------------------+------+-----+---------+----------------+
| Field  | Type                | Null | Key | Default | Extra          |
+--------+---------------------+------+-----+---------+----------------+
| cid    | tinyint(3) unsigned | NO   | PRI | NULL    | auto_increment |
| course | varchar(50)         | NO   |     | NULL    |                |
+--------+---------------------+------+-----+---------+----------------+
2 rows in set (0.01 sec)
//这种方法建立的新表和原表属性完全一样

修改表

所谓修改表，就是对已经存在的表进行操作。加、删、改->字段、索引、表名等

修改表定义：

mysql> HELP ALTER TABLE;
Name: 'ALTER TABLE'
Description:
Syntax:
ALTER [ONLINE|OFFLINE] [IGNORE] TABLE tbl_name
    [alter_specification [, alter_specification] ...]
    [partition_options]
​

对test表进行操作：

增加一个UNIQUE KEY(唯一键)：

mysql> ALTER TABLE test ADD UNIQUE KEY (course);
Query OK, 0 rows affected (0.20 sec)
Records: 0  Duplicates: 0  Warnings: 0
​
//修改字段名称
mysql> ALTER TABLE test CHANGE course Course varchar(50) NOT NULL;
Query OK, 0 rows affected (0.11 sec)
Records: 0  Duplicates: 0  Warnings: 0
​
mysql> DESC test;
+--------+---------------------+------+-----+---------+----------------+
| Field  | Type                | Null | Key | Default | Extra          |
+--------+---------------------+------+-----+---------+----------------+
| cid    | tinyint(3) unsigned | NO   | PRI | NULL    | auto_increment |
| Course | varchar(50)         | NO   | UNI | NULL    |                |
+--------+---------------------+------+-----+---------+----------------+
2 rows in set (0.00 sec)
​

给test表增加一个新字段

mysql> ALTER TABLE test ADD start_time date NOT NULL DEFAULT '2018-09-01';
Query OK, 0 rows affected (0.11 sec)
Records: 0  Duplicates: 0  Warnings: 0
mysql> DESC test;
+------------+---------------------+------+-----+------------+----------------+
| Field      | Type                | Null | Key | Default    | Extra          |
+------------+---------------------+------+-----+------------+----------------+
| cid        | tinyint(3) unsigned | NO   | PRI | NULL       | auto_increment |
| Course     | varchar(50)         | NO   | UNI | NULL       |                |
| start_time | date                | NO   |     | 2018-09-01 |                |
+------------+---------------------+------+-----+------------+----------------+
3 rows in set (0.00 sec)
​
//修改字段名称练习
mysql> ALTER TABLE test CHANGE start_time end_time date NOT NULL DEFAULT '2018-07-01';
Query OK, 0 rows affected (0.42 sec)
Records: 0  Duplicates: 0  Warnings: 0
mysql> DESC test;
+----------+---------------------+------+-----+------------+----------------+
| Field    | Type                | Null | Key | Default    | Extra          |
+----------+---------------------+------+-----+------------+----------------+
| cid      | tinyint(3) unsigned | NO   | PRI | NULL       | auto_increment |
| Course   | varchar(50)         | NO   | UNI | NULL       |                |
| end_time | date                | NO   |     | 2018-07-01 |                |
+----------+---------------------+------+-----+------------+----------------+
3 rows in set (0.00 sec)
​

给表改名：（将test给成test_new）

给表重命名代价特别大，其原理是创建一个新表，将所有数据复制进去，然后删除旧表

mysql> ALTER TABLE test RENAME test_new;
Query OK, 0 rows affected (0.00 sec)
​
mysql> SHOW TABLES;
+--------------------+
| Tables_in_students |
+--------------------+
| class_1            |
| class_2            |
| class_3            |
| courses            |
| test_new           |
| testcourses        |
+--------------------+
6 rows in set (0.00 sec)
或者直接使用`REBANE`命令
mysql> RENAME TABLE test_new TO test;//将test_new 改成 test
Query OK, 0 rows affected (0.00 sec)
mysql> SHOW TABLES; 
+--------------------+
| Tables_in_students |
+--------------------+
| class_1            |
| class_2            |
| class_3            |
| courses            |
| test               |
| testcourses        |
+--------------------+
6 rows in set (0.00 sec)
​

删除表

mysql> HELP DROP TABLE;
Name: 'DROP TABLE'
Description:
Syntax:
DROP [TEMPORARY] TABLE [IF EXISTS]
    tbl_name [, tbl_name] ...
    [RESTRICT | CASCADE]
RESTRICT是严格模式
CASCADE是即连模式，即如果有外键连接到别的表，连带删除另外的有关系的表，CASCADE是很危险的关键字

删除class_4：

mysql> DROP TABLE class_4;
Query OK, 0 rows affected (0.52 sec)
​

关于外键应用：

将courses和新创建的表classmates使用cid来连接起来

mysql> CREATE TABLE classmates(sid INT UNSIGNED NOT NULL AUTO_INCREMENT PRIMARY KEY,name     VARCHAR(30),cid INT NOT NULL); //创建一个表，字段有sid，name，和cid（cid在courses表中也有可以作为外键）
Query OK, 0 rows affected (0.83 sec)    
​
mysql> INSERT INTO classmates (name,cid) VALUES ('小明',2);   //添加数据,直接使用cid
Query OK, 1 row affected (0.16 sec)
​
mysql> INSERT INTO classmates (name,cid) VALUES ('小红',1);
Query OK, 1 row affected (0.14 sec)
​
mysql> INSERT INTO classmates (name,cid) VALUES ('范顺心',3);
Query OK, 1 row affected (0.14 sec)
​
mysql> SELECT * FROM classmates;
+-----+-----------+-----+
| sid | name      | cid |
+-----+-----------+-----+
|   1 | 小明      |   2 |
|   2 | 小红      |   1 |
|   3 | 范顺心    |   3 |
+-----+-----------+-----+
3 rows in set (0.00 sec)
mysql> SELECT * FROM courses;
+-----+--------+
| cid | course |
+-----+--------+
|   1 | 数学   |
|   2 | 语文   |
|   3 | 英语   |
+-----+--------+
3 rows in set (0.00 sec)
//组合查询方法：
mysql> SELECT name,course FROM classmates,courses WHERE classmates.cid=courses.cid;
+-----------+--------+
| name      | course |
+-----------+--------+
| 小明      | 语文   |
| 小红      | 数学   |
| 范顺心    | 英语   |
+-----------+--------+
3 rows in set (0.00 sec)
​

如果一个表中的cid超过了另一个表cid的范围，如：

mysql> INSERT INTO classmates (name,cid) VALUES ("小荒",5);
Query OK, 1 row affected (0.14 sec)
​
mysql> SELECT * FROM 
    -> classmates;
+-----+-----------+-----+
| sid | name      | cid |
+-----+-----------+-----+
|   1 | 小明      |   2 |
|   2 | 小红      |   1 |
|   3 | 范顺心    |   3 |
|   4 | 小荒      |   5 |
+-----+-----------+-----+
4 rows in set (0.00 sec)
//courses表中根本没有5cid号，如之奈何？这是需要外键约束

删除上述的例子(小荒那一行)
mysql> DELETE FROM classmates WHERE cid=5;
Query OK, 1 row affected (0.14 sec)
​
mysql> SELECT * FROM  classmates;
+-----+-----------+-----+
| sid | name      | cid |
+-----+-----------+-----+
|   1 | 小明      |   2 |
|   2 | 小红      |   1 |
|   3 | 范顺心    |   3 |
+-----+-----------+-----+
3 rows in set (0.00 sec)
​

添加外键约束：

mysql> HELP CREATE TABLE;
Name: 'CREATE TABLE'
reference_definition:
    REFERENCES tbl_name (index_col_name,...)
      [MATCH FULL | MATCH PARTIAL | MATCH SIMPLE]
      [ON DELETE reference_option]  //如果另一表删了，使用什么选项
      [ON UPDATE reference_option]  //如果另一个表更新了，使用什么选项
reference_option:
    RESTRICT | CASCADE | SET NULL | NO ACTION | SET DEFAULT //使用的选项，严格模式(不允许)|连同|设置为空|无动作|默认动作
    
mysql> ALTER TABLE classmates ADD FOREIGN KEY (cid) REFERENCES courses(cid);
ERROR 1005 (HY000): Can't create table 'students.#sql-885_4' (errno: 150)
//出现了问题，因为MyISAM(courses是MyISAM)不支持事务，所以应该修改存储引擎
mysql> ALTER TABLE courses ENGINE=InnoDB;
Query OK, 3 rows affected (0.24 sec)
Records: 3  Duplicates: 0  Warnings: 0
​

外键约束：

外键约束之前要确保属性定义需要相同

mysql> ALTER TABLE classmates MODIFY cid TINYINT UNSIGNED NOT NULL;
Query OK, 3 rows affected (0.57 sec)
Records: 3  Duplicates: 0  Warnings: 0
​
mysql> DESC courses;
+--------+---------------------+------+-----+---------+----------------+
| Field  | Type                | Null | Key | Default | Extra          |
+--------+---------------------+------+-----+---------+----------------+
| cid    | tinyint(3) unsigned | NO   | PRI | NULL    | auto_increment |
| course | varchar(50)         | NO   |     | NULL    |                |
+--------+---------------------+------+-----+---------+----------------+
2 rows in set (0.00 sec)
​
mysql> DESC classmates;
+-------+---------------------+------+-----+---------+----------------+
| Field | Type                | Null | Key | Default | Extra          |
+-------+---------------------+------+-----+---------+----------------+
| sid   | int(10) unsigned    | NO   | PRI | NULL    | auto_increment |
| name  | varchar(30)         | YES  |     | NULL    |                |
| cid   | tinyint(3) unsigned | NO   |     | NULL    |                |
+-------+---------------------+------+-----+---------+----------------+
3 rows in set (0.00 sec)
​
mysql> ALTER TABLE classmates ADD FOREIGN KEY foreign_cid(cid) REFERENCES courses (cid);//添加外键约束，给classmates表添加courses的约束（cid相同）
Query OK, 3 rows affected (0.58 sec)
Records: 3  Duplicates: 0  Warnings: 0
​
mysql> SHOW INDEXES FROM classmates\G
*************************** 2. row ***************************
        Table: classmates
   Non_unique: 1
     Key_name: foreign_cid      //表明是外键约束
 Seq_in_index: 1
  Column_name: cid
    Collation: A
  Cardinality: 3
     Sub_part: NULL
       Packed: NULL
         Null: 
   Index_type: BTREE
      Comment: 
Index_comment: 
2 rows in set (0.00 sec)
​
mysql> INSERT INTO classmates (name,cid) VALUE ('小荒',4);//courses表中没有cid=4，则在classmates不能添加cid=4，即外键约束
ERROR 1452 (23000): Cannot add or update a child row: a foreign key constraint fails (`students`.`classmates`, CONSTRAINT `classmates_ibfk_1` FOREIGN KEY (`cid`) REFERENCES `courses` (`cid`))
mysql> DELETE FROM courses WHERE cid=2; //同样不允删除正在被其他表引用的外键约束值
ERROR 1451 (23000): Cannot delete or update a parent row: a foreign key constraint fails (`students`.`classmates`, CONSTRAINT `classmates_ibfk_1` FOREIGN KEY (`cid`) REFERENCES `courses` (`cid`))

总结注意：外键和存储引擎有关系，需要InnoDB存储模式外键约束极大的消耗资源，所以不建议使用。

索引

索引不能被修改，只能创建，删除，或者查看，索引本身是一个解构体实例化

创建索引

CREATE INDEX index_name ON tb_name (字段col);

col_name length [ASC | DESC]

length指定索引长度，只索引一个字段的多少个字符值需要合理，有一定的计算法则

ASC表示升序，DESC表示降序

mysql> HELP CREATE INDEX
Name: 'CREATE INDEX'
Description:
Syntax:
CREATE [ONLINE|OFFLINE] [UNIQUE|FULLTEXT|SPATIAL] INDEX index_name
    [index_type]
    ON tbl_name (index_col_name,...)
    [index_option] ...
​
mysql> SHOW INDEXES FROM classmates;
+------------+------------+-------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
| Table      | Non_unique | Key_name    | Seq_in_index | Column_name | Collation | Cardinality | Sub_part | Packed | Null | Index_type | Comment | Index_comment |
+------------+------------+-------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
| classmates |          0 | PRIMARY     |            1 | sid         | A         |           4 |     NULL | NULL   |      | BTREE      |         |               |
| classmates |          1 | foreign_cid |            1 | cid         | A         |           4 |     NULL | NULL   |      | BTREE      |         |               |
+------------+------------+-------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
2 rows in set (0.00 sec)
​

mysql> CREATE INDEX name_on_classmates ON classmates (name) USING BTREE
    -> ;
Query OK, 0 rows affected (0.35 sec)
Records: 0  Duplicates: 0  Warnings: 0
​
mysql> SHOW INDEXES FROM classmates \G
*************************** 3. row ***************************
        Table: classmates
   Non_unique: 1
     Key_name: name_on_classmates
 Seq_in_index: 1
  Column_name: name
    Collation: A
  Cardinality: 4
     Sub_part: NULL
       Packed: NULL
         Null: YES
   Index_type: BTREE
      Comment: 
Index_comment: 
3 rows in set (0.00 sec)

删除索引

删除索引使用DROP INDEX

mysql> HELP DROP INDEX;
Name: 'DROP INDEX'
Description:
Syntax:
DROP [ONLINE|OFFLINE] INDEX index_name ON tbl_name
mysql> SHOW INDEXES FROM classmates \G
*************************** 1. row ***************************
        Table: classmates
   Non_unique: 0
     Key_name: PRIMARY
 Seq_in_index: 1
  Column_name: sid
    Collation: A
  Cardinality: 4
     Sub_part: NULL
       Packed: NULL
         Null: 
   Index_type: BTREE
      Comment: 
Index_comment: 
*************************** 2. row ***************************
        Table: classmates
   Non_unique: 1
     Key_name: foreign_cid
 Seq_in_index: 1
  Column_name: cid
    Collation: A
  Cardinality: 4
     Sub_part: NULL
       Packed: NULL
         Null: 
   Index_type: BTREE
      Comment: 
Index_comment: 
2 rows in set (0.00 sec)
​

删除索引：

mysql> DROP INDEX name_on_classmates ON classmates;
Query OK, 0 rows affected (0.19 sec)
Records: 0  Duplicates: 0  Warnings: 0
​