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007-LAMP_MySQL_DDL

2016-01-19 17:05 543 查看

007-LAMP_MySQL_datatype && DDL

MySQL数据类型

MySQL支持多种类型，大致可以分为三类：数值、日期/时间和字符串(字符)类型。

数值类型

MySQL支持所有标准SQL数值数据类型。

这些类型包括严格数值数据类型(INTEGER、SMALLINT、DECIMAL和NUMERIC)，以及近似数值数据类型(FLOAT、REAL和DOUBLE PRECISION)。

关键字INT是INTEGER的同义词，关键字DEC是DECIMAL的同义词。

BIT数据类型保存位字段值，并且支持MyISAM、MEMORY、InnoDB和BDB表。

作为SQL标准的扩展，MySQL也支持整数类型TINYINT、MEDIUMINT和BIGINT。下面的表显示了需要的每个整数类型的存储和范围。

类型
大小
范围（有符号）
范围（无符号）
用途
TINYINT
1 字节
(-128，127)
(0，255)
小整数值
SMALLINT
2 字节
(-32 768，32 767)
(0，65 535)
大整数值
MEDIUMINT
3 字节
(-8 388 608，8 388 607)
(0，16 777 215)
大整数值
INT或INTEGER
4 字节
(-2 147 483 648，2 147 483 647)
(0，4 294 967 295)
大整数值
BIGINT
8 字节
(-9 233 372 036 854 775 808，9 223 372 036 854 775 807)
(0，18 446 744 073 709 551 615)
极大整数值
FLOAT
4 字节
(-3.402 823 466 E+38，1.175 494 351 E-38)，0，(1.175 494 351 E-38，3.402 823 466 351 E+38)
0，(1.175 494 351 E-38，3.402 823 466 E+38)
单精度
浮点数值
DOUBLE
8 字节
(1.797 693 134 862 315 7 E+308，2.225 073 858 507 201 4 E-308)，0，(2.225 073 858 507 201 4 E-308，1.797 693 134 862 315 7 E+308)
0，(2.225 073 858 507 201 4 E-308，1.797 693 134 862 315 7 E+308)
双精度
浮点数值
DECIMAL
对DECIMAL(M,D) ，如果M>D，为M+2否则为D+2
依赖于M和D的值
依赖于M和D的值
小数值

日期和时间类型

表示时间值的日期和时间类型为DATETIME、DATE、TIMESTAMP、TIME和YEAR。

每个时间类型有一个有效值范围和一个”零”值，当指定不合法的MySQL不能表示的值时使用”零”值。

TIMESTAMP类型有专有的自动更新特性，将在后面描述。

类型
大小
(字节)
范围
格式
用途
DATE
3
1000-01-01/9999-12-31
YYYY-MM-DD
日期值
TIME
3
‘-838:59:59’/’838:59:59’
HH:MM:SS
时间值或持续时间
YEAR
1
1901/2155
YYYY
年份值
DATETIME
8
1000-01-01 00:00:00/9999-12-31 23:59:59
YYYY-MM-DD HH:MM:SS
混合日期和时间值
TIMESTAMP
8
1970-01-01 00:00:00/2037 年某时
YYYYMMDD HHMMSS
混合日期和时间值，时间戳

字符串类型

字符串类型指CHAR、VARCHAR、BINARY、VARBINARY、BLOB、TEXT、ENUM和SET。该节描述了这些类型如何工作以及如何在查询中使用这些类型。

类型
大小
用途
CHAR
0-255字节
定长字符串
VARCHAR
0-65535 字节
变长字符串
TINYBLOB
0-255字节
不超过 255 个字符的二进制字符串
TINYTEXT
0-255字节
短文本字符串
BLOB
0-65 535字节
二进制形式的长文本数据
TEXT
0-65 535字节
长文本数据
MEDIUMBLOB
0-16 777 215字节
二进制形式的中等长度文本数据
MEDIUMTEXT
0-16 777 215字节
中等长度文本数据
LOGNGBLOB
0-4 294 967 295字节
二进制形式的极大文本数据
LONGTEXT
0-4 294 967 295字节
极大文本数据
CHAR和VARCHAR类型类似，但它们保存和检索的方式不同。它们的最大长度和是否尾部空格被保留等方面也不同。在存储或检索过程中不进行大小写转换。

BINARY和VARBINARY类类似于CHAR和VARCHAR，不同的是它们包含二进制字符串而不要非二进制字符串。也就是说，它们包含字节字符串而不是字符字符串。这说明它们没有字符集，并且排序和比较基于列值字节的数值值。

BLOB是一个二进制大对象，可以容纳可变数量的数据。有4种BLOB类型：TINYBLOB、BLOB、MEDIUMBLOB和LONGBLOB。它们只是可容纳值的最大长度不同。

有4种TEXT类型：TINYTEXT、TEXT、MEDIUMTEXT和LONGTEXT。这些对应4种BLOB类型，有相同的最大长度和存储需求。

注意：数据类型有修饰符概念

UNSIGNED：无符号

NOT NULL：非空

DEFAULT value：默认值

服务端命令

通过mysql client连接发往mysql server执行并取回结果

DDL、DML、DCL
主语：每个语句必须有语句结束符，默认为分号(;)


DDL

数据库管理

创建数据库

Syntax:
CREATE {DATABASE | SCHEMA} [IF NOT EXISTS] db_name
[create_specification] ...

create_specification:
[DEFAULT] CHARACTER SET [=] charset_name    #指明字符集
| [DEFAULT] COLLATE [=] collation_name        #指明排序规则

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.


查看支持字符集

MariaDB [(none)]> SHOW CHARACTER SET;

查看排序规则

MariaDB [(none)]> SHOW COLLATION;

-

MariaDB [(none)]> CREATE DATABASE sdb;
Query OK, 1 row affected (0.00 sec)

MariaDB [(none)]> SHOW DATABASES;
+--------------------+
| Database           |
+--------------------+
| information_schema |
| db01               |
| mysql              |
| performance_schema |
| sdb                |
| test               |
| wordpress          |
+--------------------+
7 rows in set (0.00 sec)


执行后发现多了一个数据库 sdb 。

修改数据库

Syntax:
ALTER {DATABASE | SCHEMA} [db_name]
alter_specification ...
ALTER {DATABASE | SCHEMA} db_name
UPGRADE DATA DIRECTORY NAME

alter_specification:
[DEFAULT] CHARACTER SET [=] charset_name    #修改字符集
| [DEFAULT] COLLATE [=] collation_name        #修改排序规则

ALTER DATABASE enables you to change the overall characteristics of a
data
4000
base. These characteristics are stored in the db.opt file in the
database directory. To use ALTER DATABASE, you need the ALTER privilege
on the database. ALTER SCHEMA is a synonym for ALTER DATABASE.


删除数据库

Syntax:
DROP {DATABASE | SCHEMA} [IF EXISTS] db_name

DROP DATABASE drops all tables in the database and deletes the
database. Be very careful with this statement! To use DROP DATABASE,
you need the DROP privilege on the database. DROP SCHEMA is a synonym
for DROP DATABASE.

Important: When a database is dropped, user privileges on the
database are not automatically dropped. See [HELP GRANT].

IF EXISTS is used to prevent an error from occurring if the database
does not exist.


MariaDB [(none)]> DROP DATABASE IF EXISTS sdb;


执行上述命令就删除了创建的数据库 sdb ；

表管理

创建表，此处是直接创建，还有复制表数据和表结构两种创建方式

Syntax:
CREATE [TEMPORARY] TABLE [IF NOT EXISTS] tbl_name
(create_definition,...)
[table_options]
[partition_options]

create_definition:
col_name column_definition
| [CONSTRAINT [symbol]] PRIMARY KEY [index_type] (index_col_name,...)
[index_option] ...
| {INDEX|KEY} [index_name] [index_type] (index_col_name,...)
[index_option] ...
| [CONSTRAINT [symbol]] UNIQUE [INDEX|KEY]
[index_name] [index_type] (index_col_name,...)
[index_option] ...
| {FULLTEXT|SPATIAL} [INDEX|KEY] [index_name] (index_col_name,...)
[index_option] ...
| [CONSTRAINT [symbol]] FOREIGN KEY
[index_name] (index_col_name,...) reference_definition
| CHECK (expr)

column_definition:
data_type [NOT NULL | NULL] [DEFAULT default_value]
[AUTO_INCREMENT] [UNIQUE [KEY] | [PRIMARY] KEY]
[COMMENT 'string']
[COLUMN_FORMAT {FIXED|DYNAMIC|DEFAULT}]
[STORAGE {DISK|MEMORY|DEFAULT}]
[reference_definition]

index_col_name:
col_name [(length)] [ASC | DESC]

index_type:
USING {BTREE | HASH}

index_option:
KEY_BLOCK_SIZE [=] value
| index_type
| WITH PARSER parser_name
| COMMENT 'string'

reference_definition:
REFERENCES tbl_name (index_col_name,...)
[MATCH FULL | MATCH PARTIAL | MATCH SIMPLE]
[ON DELETE reference_option]
[ON UPDATE reference_option]

reference_option:
RESTRICT | CASCADE | SET NULL | NO ACTION

table_options:
table_option [[,] table_option] ...

table_option:
ENGINE [=] engine_name

partition_options:
PARTITION BY
{ [LINEAR] HASH(expr)
| [LINEAR] KEY(column_list)
| RANGE{(expr) | COLUMNS(column_list)}
| LIST{(expr) | COLUMNS(column_list)} }
[PARTITIONS num]
[SUBPARTITION BY
{ [LINEAR] HASH(expr)
| [LINEAR] KEY(column_list) }
[SUBPARTITIONS num]
]
[(partition_definition [, partition_definition] ...)]

CREATE TABLE creates a table with the given name. You must have the
CREATE privilege for the table.


create table命令选项巨多，简要格式

CREATE TABLE [IF NOT EXISTS] tlb_name (create_defination) [table options]
create_defination:
字段：col_name column_defination
键：PRIMARY KEY(col1,col2,...)
UNIQUE KEY(col1mcol2,...)
FOREIGN KEY(column)
索引：
KEY|INDEX [index_name] (col1,col2,...)
table_options:
存储引擎：ENGINE=


创建表格

MariaDB [(none)]> use sdb;
Database changed

MariaDB [sdb]> CREATE TABLE students(id INT UNSIGNED NOT NULL PRIMARY KEY,name CHAR(30) NOT NULL,age TINYINT UNSIGNED,sex ENUM('F','M'));
Query OK, 0 rows affected (0.62 sec)

MariaDB [sdb]> DESC students;
+-------+---------------------+------+-----+---------+-------+
| Field | Type                | Null | Key | Default | Extra |
+-------+---------------------+------+-----+---------+-------+
| id    | int(10) unsigned    | NO   | PRI | NULL    |       |
| name  | char(30)            | NO   |     | NULL    |       |
| age   | tinyint(3) unsigned | YES  |     | NULL    |       |
| sex   | enum('F','M')       | YES  |     | NULL    |       |
+-------+---------------------+------+-----+---------+-------+
4 rows in set (0.00 sec)


定义多个主键

MariaDB [sdb]> CREATE TABLE students(id INT UNSIGNED NOT NULL,name CHAR(30) NOT NULL,age TINYINT UNSIGNED,sex ENUM('F','M'),PRIMARY KEY(id,name));
Query OK, 0 rows affected (0.02 sec)

MariaDB [sdb]> DESC students;
+-------+---------------------+------+-----+---------+-------+
| Field | Type                | Null | Key | Default | Extra |
+-------+---------------------+------+-----+---------+-------+
| id    | int(10) unsigned    | NO   | PRI | NULL    |       |
| name  | char(30)            | NO   | PRI | NULL    |       |
| age   | tinyint(3) unsigned | YES  |     | NULL    |       |
| sex   | enum('F','M')       | YES  |     | NULL    |       |
+-------+---------------------+------+-----+---------+-------+
4 rows in set (0.00 sec)


注意：UNSIGNED这个一定要紧跟在INT后面

查看支持的存储引擎

MariaDB [sdb]> SHOW ENGINES;


查看某表的状态(存储引擎、字符集等)

MariaDB [sdb]> SHOW TABLE STATUS like 'students'\G;
*************************** 1. row ***************************
Name: students
Engine: InnoDB
Version: 10
Row_format: Compact
Rows: 0
Avg_row_length: 0
Data_length: 16384
Max_data_length: 0
Index_length: 0
Data_free: 6291456
Auto_increment: NULL
Create_time: 2016-01-18 08:29:28
Update_time: NULL
Check_time: NULL
Collation: latin1_swedish_ci
Checksum: NULL
Create_options:
Comment:


修改表

Syntax:
ALTER [ONLINE | OFFLINE] [IGNORE] TABLE tbl_name
[alter_specification [, alter_specification] ...]
[partition_options]

alter_specification:
table_options
| ADD [COLUMN] col_name column_definition
[FIRST | AFTER col_name ]
| ADD [COLUMN] (col_name column_definition,...)
| ADD {INDEX|KEY} [index_name]
[index_type] (index_col_name,...) [index_option] ...
| ADD [CONSTRAINT [symbol]] PRIMARY KEY
[index_type] (index_col_name,...) [index_option] ...
| ADD [CONSTRAINT [symbol]]
UNIQUE [INDEX|KEY] [index_name]
[index_type] (index_col_name,...) [index_option] ...
| ADD FULLTEXT [INDEX|KEY] [index_name]
(index_col_name,...) [index_option] ...
| ADD SPATIAL [INDEX|KEY] [index_name]
(index_col_name,...) [index_option] ...
| ADD [CONSTRAINT [symbol]]
FOREIGN KEY [index_name] (index_col_name,...)
reference_definition
| ALTER [COLUMN] col_name {SET DEFAULT literal | DROP DEFAULT}
| CHANGE [COLUMN] old_col_name new_col_name column_definition
[FIRST|AFTER col_name]
| MODIFY [COLUMN] col_name column_definition
[FIRST | AFTER col_name]
| DROP [COLUMN] col_name
| DROP PRIMARY KEY
| DROP {INDEX|KEY} index_name
| DROP FOREIGN KEY fk_symbol
| MAX_ROWS = rows
| DISABLE KEYS
| ENABLE KEYS
| RENAME [TO|AS] new_tbl_name
| ORDER BY col_name [, col_name] ...
| CONVERT TO CHARACTER SET charset_name [COLLATE collation_name]
| [DEFAULT] CHARACTER SET [=] charset_name [COLLATE [=] collation_name]
| DISCARD TABLESPACE
| IMPORT TABLESPACE
| FORCE
| ADD PARTITION (partition_definition)
| DROP PARTITION partition_names
| TRUNCATE PARTITION {partition_names | ALL}
| COALESCE PARTITION number
| REORGANIZE PARTITION [partition_names INTO (partition_definitions)]
| ANALYZE PARTITION {partition_names | ALL}
| CHECK PARTITION {partition_names | ALL}
| OPTIMIZE PARTITION {partition_names | ALL}
| REBUILD PARTITION {partition_names | ALL}
| REPAIR PARTITION {partition_names | ALL}
| PARTITION BY partitioning_expression
| REMOVE PARTITIONING

index_col_name:
col_name [(length)] [ASC | DESC]

index_type:
USING {BTREE | HASH}

index_option:
KEY_BLOCK_SIZE [=] value
| index_type
| WITH PARSER parser_name
| COMMENT 'string'

table_options:
table_option [[,] table_option] ...  (see CREATE TABLE options)

partition_options:
(see CREATE TABLE options)

ALTER TABLE changes the structure of a table. For example, you can add
or delete columns, create or destroy indexes, change the type of
existing columns, or rename columns or the table itself. You can also
change characteristics such as the storage engine used for the table or
the table comment.

Partitioning-related clauses for ALTER TABLE can be used with
partitioned tables for repartitioning, for adding, dropping, merging,
and splitting partitions, and for performing partitioning maintenance.


alter table命令选项巨多，简要格式

ALTER [ONLINE | OFFLINE] [IGNORE] TABLE tbl_name

[alter_specification [, alter_specification] …]

[partition_options]

alter_specification：

字段：

添加：ADD [COLUMN] col_name column_definition [FIRST | AFTER col_name ]

FIRST | AFTER col_name表示把添加的字段放在整张表中的什么位置。

删除：DROP [COLUMN] col_name

修改：CHANGE [COLUMN] old_col_name new_col_name column_definition [FIRST|AFTER col_name]

MODIFY [COLUMN] col_name column_definition [FIRST | AFTER col_name]

CHANGE修改整个字段、MODIFY修改字段属性

键：
添加：ADD {PRIMARY | UNIQUE | FOREIGN} KEY {col1,col2,...}
删除：DROP PRIMARY KEY | DROP FOREIGN KEY fk_symblo
索引：
添加：ADD INDEX [index_name] {col1,col2,...}   #即使不定义名字，默认也会有索引的名字
删除：DROP {INDEX | KEY} index_name
引擎：
ENGINE [=] engine_name


添加字段并放在name字段之后

MariaDB [sdb]> DESC students;
+-------+---------------------+------+-----+---------+-------+
| Field | Type                | Null | Key | Default | Extra |
+-------+---------------------+------+-----+---------+-------+
| id    | int(10) unsigned    | NO   | PRI | NULL    |       |
| name  | char(30)            | NO   | PRI | NULL    |       |
| age   | tinyint(3) unsigned | YES  |     | NULL    |       |
| sex   | enum('F','M')       | YES  |     | NULL    |       |
+-------+---------------------+------+-----+---------+-------+
4 rows in set (0.00 sec)

MariaDB [sdb]> ALTER TABLE students ADD class VARCHAR(100) NOT NULL AFTER name;
Query OK, 0 rows affected (0.05 sec)
Records: 0  Duplicates: 0  Warnings: 0

MariaDB [sdb]> DESC students;
+-------+---------------------+------+-----+---------+-------+
| Field | Type                | Null | Key | Default | Extra |
+-------+---------------------+------+-----+---------+-------+
| id    | int(10) unsigned    | NO   | PRI | NULL    |       |
| name  | char(30)            | NO   | PRI | NULL    |       |
| class | varchar(100)        | NO   |     | NULL    |       |
| age   | tinyint(3) unsigned | YES  |     | NULL    |       |
| sex   | enum('F','M')       | YES  |     | NULL    |       |
+-------+---------------------+------+-----+---------+-------+
5 rows in set (0.00 sec)


在name字段上添加唯一键

MariaDB [sdb]> DESC students;
+-------+---------------------+------+-----+---------+-------+
| Field | Type                | Null | Key | Default | Extra |
+-------+---------------------+------+-----+---------+-------+
| id    | int(10) unsigned    | NO   | PRI | NULL    |       |
| name  | char(30)            | NO   | PRI |
a295
NULL    |       |
| class | varchar(100)        | NO   |     | NULL    |       |
| age   | tinyint(3) unsigned | YES  |     | NULL    |       |
| sex   | enum('F','M')       | YES  |     | NULL    |       |
+-------+---------------------+------+-----+---------+-------+
5 rows in set (0.00 sec)

MariaDB [sdb]> ALTER TABLE students DROP PRIMARY KEY;
Query OK, 0 rows affected (0.01 sec)
Records: 0  Duplicates: 0  Warnings: 0

MariaDB [sdb]> ALTER TABLE students ADD UNIQUE KEY (name);
Query OK, 0 rows affected (0.14 sec)
Records: 0  Duplicates: 0  Warnings: 0

MariaDB [sdb]> DESC students;
+-------+---------------------+------+-----+---------+-------+
| Field | Type                | Null | Key | Default | Extra |
+-------+---------------------+------+-----+---------+-------+
| id    | int(10) unsigned    | NO   |     | NULL    |       |
| name  | char(30)            | NO   | PRI | NULL    |       |
| class | varchar(100)        | NO   |     | NULL    |       |
| age   | tinyint(3) unsigned | YES  |     | NULL    |       |
| sex   | enum('F','M')       | YES  |     | NULL    |       |
+-------+---------------------+------+-----+---------+-------+
5 rows in set (0.00 sec)


在上例中由于name NOT NULL，因此其唯一键会被标识为PRI，但是删除的时候一定要用INDEX或KEY来删除

MariaDB [sdb]> desc students;
+-------+---------------------+------+-----+---------+-------+
| Field | Type                | Null | Key | Default | Extra |
+-------+---------------------+------+-----+---------+-------+
| id    | int(10) unsigned    | NO   |     | NULL    |       |
| name  | char(30)            | NO   | PRI | NULL    |       |
| class | varchar(100)        | NO   |     | NULL    |       |
| age   | tinyint(3) unsigned | YES  |     | NULL    |       |
| sex   | enum('F','M')       | YES  |     | NULL    |       |
+-------+---------------------+------+-----+---------+-------+
5 rows in set (0.00 sec)

MariaDB [sdb]> ALTER TABLE students DROP INDEX name;
Query OK, 0 rows affected (0.00 sec)
Records: 0  Duplicates: 0  Warnings: 0


查看表中的索引信息

MariaDB [sdb]> SHOW INDEXES FROM students\G;
*************************** 1. row ***************************
Table: students
Non_unique: 0
Key_name: name
Seq_in_index: 1
Column_name: name
Collation: A
Cardinality: 0
Sub_part: NULL
Packed: NULL
Null:
Index_type: BTREE
Comment:
Index_comment:
1 row in set (0.00 sec)

ERROR: No query specified


添加索引，索引名字为sindex，且索引字段在class和id上

MariaDB [sdb]> DESC students;
+-------+---------------------+------+-----+---------+-------+
| Field | Type                | Null | Key | Default | Extra |
+-------+---------------------+------+-----+---------+-------+
| id    | int(10) unsigned    | NO   |     | NULL    |       |
| name  | char(30)            | NO   | PRI | NULL    |       |
| class | varchar(100)        | NO   |     | NULL    |       |
| age   | tinyint(3) unsigned | YES  |     | NULL    |       |
| sex   | enum('F','M')       | YES  |     | NULL    |       |
+-------+---------------------+------+-----+---------+-------+
5 rows in set (0.00 sec)

MariaDB [sdb]> ALTER TABLE students ADD INDEX sindex (id,class);
Query OK, 0 rows affected (0.01 sec)
Records: 0  Duplicates: 0  Warnings: 0

MariaDB [sdb]> SHOW INDEX FROM students\g;
+----------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
| Table    | Non_unique | Key_name | Seq_in_index | Column_name | Collation | Cardinality | Sub_part | Packed | Null | Index_type | Comment | Index_comment |
+----------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
| students |          0 | name     |            1 | name        | A         |           0 |     NULL | NULL   |      | BTREE      |         |               |
| students |          1 | sindex   |            1 | id          | A         |           0 |     NULL | NULL   |      | BTREE      |         |               |
| students |          1 | sindex   |            2 | class       | A         |           0 |     NULL | NULL   |      | BTREE      |         |               |
+----------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
3 rows in set (0.00 sec)


显示索引信息

MariaDB [sdb]> ALTER TABLE students DROP INDEX sindex;
Query OK, 0 rows affected (0.00 sec)
Records: 0  Duplicates: 0  Warnings: 0

MariaDB [sdb]> SHOW INDEXES FROM students;
+----------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
| Table    | Non_unique | Key_name | Seq_in_index | Column_name | Collation | Cardinality | Sub_part | Packed | Null | Index_type | Comment | Index_comment |
+----------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
| students |          0 | name     |            1 | name        | A         |           0 |     NULL | NULL   |      | BTREE      |         |               |
+----------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
1 row in set (0.00 sec)


删除表

Syntax:

DROP [TEMPORARY] TABLE [IF EXISTS]

tbl_name [, tbl_name] …

[RESTRICT | CASCADE]

DROP TABLE removes one or more tables. You must have the DROP privilege

for each table. All table data and the table definition are removed, so

be careful with this statement! If any of the tables named in the

argument list do not exist, MySQL returns an error indicating by name

which nonexisting tables it was unable to drop, but it also drops all

of the tables in the list that do exist.

索引管理

索引类型有HASH、BTREE等

创建索引

Syntax:
CREATE [ONLINE|OFFLINE] [UNIQUE|FULLTEXT|SPATIAL] INDEX index_name
[index_type]
ON tbl_name (index_col_name,...)
[index_option] ...

index_col_name:
col_name [(length)] [ASC | DESC]

index_type:
USING {BTREE | HASH}

index_option:
KEY_BLOCK_SIZE [=] value
| index_type
| WITH PARSER parser_name
| COMMENT 'string'

CREATE INDEX is mapped to an ALTER TABLE statement to create indexes.
See [HELP ALTER TABLE]. CREATE INDEX cannot be used to create a PRIMARY
KEY; use ALTER TABLE instead. For more information about indexes, see http://dev.mysql.com/doc/refman/5.5/en/mysql-indexes.html.


在students这张表中的id和class字段创建(多值)索引

MariaDB [sdb]> DESC students;
+-------+---------------------+------+-----+---------+-------+
| Field | Type                | Null | Key | Default | Extra |
+-------+---------------------+------+-----+---------+-------+
| id    | int(10) unsigned    | NO   |     | NULL    |       |
| name  | char(30)            | NO   |     | NULL    |       |
| class | varchar(100)        | NO   |     | NULL    |       |
| age   | tinyint(3) unsigned | YES  |     | NULL    |       |
| sex   | enum('F','M')       | YES  |     | NULL    |       |
+-------+---------------------+------+-----+---------+-------+
5 rows in set (0.00 sec)

MariaDB [sdb]> CREATE INDEX sindex ON students (id,class);
Query OK, 0 rows affected (0.01 sec)
Records: 0  Duplicates: 0  Warnings: 0

MariaDB [sdb]> SHOW INDEXES FROM students;
+----------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
| Table    | Non_unique | Key_name | Seq_in_index | Column_name | Collation | Cardinality | Sub_part | Packed | Null | Index_type | Comment | Index_comment |
+----------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
| students |          1 | sindex   |            1 | id          | A         |           0 |     NULL | NULL   |      | BTREE      |         |               |
| students |          1 | sindex   |            2 | class       | A         |           0 |     NULL | NULL   |      | BTREE      |         |               |
+----------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
2 rows in set (0.00 sec)


删除索引

Syntax:
DROP [ONLINE|OFFLINE] INDEX index_name ON tbl_name

DROP INDEX drops the index named index_name from the table tbl_name.
This statement is mapped to an ALTER TABLE statement to drop the index.
See [HELP ALTER TABLE].

To drop a primary key, the index name is always PRIMARY, which must be
specified as a quoted identifier because PRIMARY is a reserved word:

DROP INDEX `PRIMARY` ON t;


删除上面创建的索引

MariaDB [sdb]> DROP INDEX sindex ON students;
Query OK, 0 rows affected (0.00 sec)
Records: 0  Duplicates: 0  Warnings: 0
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标签：  mysql DDL
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