MySQL 数据库SQL语句---DDL语句

SQL语句---DDL语句

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概述：

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MySQL服务端SQL语句 

   ---服务端命令：SQL语句，发往服务端运行，并取回结果；需要显式的语句结束符；

★DDL：数据定义语言，

☉作用：

• 主要用于数据库组件，例如数据库、表、索引、视图、触发器、事件调度器、存储过程、存储函数；

☉常用命令：

• CREATE（创建）， ALTER（修改）， DROP（删除）（？后跟命令可获取帮助）

★DML：数据操纵语言

☉作用：

• CRUD（增删改查）操作，主要用于操作表中的数据；每一种操作之前都要先查询；

☉命令

• INSERT，DELETE，UPDATE，SELECT

★DCL：数据控制语言

☉作用：

• 授权用户，登录主机地址权限及回收权限

☉命令

• GRANT（授权）, REVOKE（回收权限）

  SQL MODE：定义mysqld对约束等违反时的响应行为等设定；

★常用的MODE：

• TRADITioNAL ：         传统的模式，违反数据定义的统统都不被允许；

• STRICT_TRANS_TABLES ： 仅对事物型表严格限定；

• STRICT_ALL_TABLES ：   对所有的表都做严格限定；

★修改方式：

• Mysql> SET GLOBAL sql_mode='MODE';

• mysql> SET @@global.sql_mode='MODE';

注意：

• 默认为空模式，如果违反数据定义，会发出警报，会以允许的最大范围去修减数据

• sql mode为必改参数，要想永久生效，要写入配置文件

演示：

 1.在sql mode模式为空的时候（默认），向表中插入数据，可以插入成功，但对违反数据定义的会对数据进行修减到允许的最大范围，如下：

MariaDB [(none)]> SELECT @@session.sql_mode;
+--------------------+
| @@session.sql_mode |
+--------------------+
|                    |
+--------------------+
1 row in set (0.00 sec)

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

MariaDB [(none)]> use testdb;
Database changed
MariaDB [testdb]> create table tbl1(id tinyint unsigned,name CHAR(5));
Query OK, 0 rows affected (0.03 sec)

MariaDB [testdb]> insert into tbl1 (id) values (16),(256); # 默认最大为255
Query OK, 2 rows affected, 1 warning (0.00 sec) # 报错
Records: 2  Duplicates: 0  Warnings: 1

MariaDB [testdb]> select * from tbl1;
+------+------+
| id   | name |
+------+------+
|   16 | NULL |
|  255 | NULL |  # 可以发现我们插入的256没有成功，只到允许插入的最大范围
+------+------+
2 rows in set (0.00 sec)

MariaDB [testdb]> insert into tbl1 (name) values ('jerry'),('taotaoxiuxiu');
Query OK, 2 rows affected, 1 warning (0.00 sec)
Records: 2  Duplicates: 0  Warnings: 1

MariaDB [testdb]> show Warnings;
+---------+------+-------------------------------------------+
| Level   | Code | Message                                   |
+---------+------+-------------------------------------------+
| Warning | 1265 | Data truncated for column 'name' at row 2 |
+---------+------+-------------------------------------------+
1 row in set (0.00 sec)

MariaDB [testdb]> select * from tbl1;
+------+-------+
| id   | name  |
+------+-------+
|   16 | NULL  |
|  255 | NULL  |
| NULL | jerry |
| NULL | taota |  # 我们定义的最大只能插入5个字符，多以多出来的将会被修减
+------+-------+
4 rows in set (0.00 sec)

  2.现在我们定义sql mode模式为TRADITIONAL（传统模式），即对数据进行严格的限定，对违反数据要求的统统不予许插入，如下：

MariaDB [testdb]> SET @@session.sql_mode='TRADITIONAL'; # 设定当前会话为传统模式；
Query OK, 0 rows affected (0.00 sec)

MariaDB [testdb]> SELECT @@session.sql_mode;
+------------------------------------------------------------------------------------------------------------------------------------------------------+
| @@session.sql_mode                                                                                                                                   |
+------------------------------------------------------------------------------------------------------------------------------------------------------+
| STRICT_TRANS_TABLES,STRICT_ALL_TABLES,NO_ZERO_IN_DATE,NO_ZERO_DATE,ERROR_FOR_DIVISION_BY_ZERO,TRADITIONAL,NO_AUTO_CREATE_USER,NO_ENGINE_SUBSTITUTION |
+------------------------------------------------------------------------------------------------------------------------------------------------------+
1 row in set (0.00 sec)

MariaDB [testdb]> insert into tbl1 (name) values ('jerry'),('taotaoxiuxiu');
ERROR 1406 (22001): Data too long for column 'name' at row 2  # 再次插入报错，不允许插入

SQL语句之DDL语句

 1.获取帮助

• mysql> help KEYWord
  

• mysql> help contents

演示：

MariaDB [(none)]> help contents
You asked for help about help cateGory: "Contents"
For more infORMation, type 'help <item>', where <item> is one of the following
categories:
   Account Management
   Administration
   Compound Statements
   Data Definition
   Data Manipulation
   Data Types
   Functions
   Functions and Modifiers for Use with GROUP BY
   Geographic Features
   Help Metadata
   Language Structure
   Plugins
   Procedures
   Table Maintenance
   Transactions
   User-Defined Functions
   Utility

MariaDB [(none)]> help Data Types # 获取数据类型
You asked for help about help category: "Data Types"
For more information, type 'help <item>', where <item> is one of the following
topics:
   AUTO_INCREMENT
   BIGINT
   BINARY
   BIT
   BLOB
   BLOB DATA TYPE
   BOOLEAN
   CHAR
   CHAR BYTE
   DATE
   DATETIME
   DEC
   DECIMAL
   DOUBLE
   DOUBLE PRECISION
   ENUM
   FLOAT
   INT
   INTEGER
   LONGBLOB
   LONGTEXT
   MEDIUMBLOB
   MEDIUMINT
   MEDIUMTEXT
   SET DATA TYPE
   SMALLINT
   TEXT
   TIME
   TIMESTAMP
   TINYBLOB
   TINYINT
   TINYTEXT
   VARBINARY
   VARCHAR
   YEAR DATA TYPE

MariaDB [(none)]> help INT
Name: 'INT'
Description:
INT[(M)] [UNSIGNED] [ZEROFILL]

A normal-size integer. The signed range is -2147483648 to 2147483647.
The unsigned range is 0 to 4294967295.

URL: Http://dev.mysql.com/doc/refman/5.5/en/numeric-type-overview.html

 2.数据库管理

★创建数据库：

• CREATE {DATABASE | SCHEMA} [IF NOT EXISTS] db_name  [DEFAULT] CHARACTER SET [=] charset_name 

• character 字符集，SHOW CHARACTER SET 可查看所支持的字符集 

★修改数据库

• ALTER {DATABASE | SCHEMA} [db_name] CHARACTER SET [=] charset_name

★删除数据库

• DROP {DATABASE | SCHEMA} [IF EXISTS] db_name 

★相关命令：

• SHOW CHARACTER SET   //查看字符集；

• SHOW COLLATION          //查看排序规则；

• SHOW CREATE DATABASE db_name    //查看创建数据库时所使用的语句；

命令演示：

MariaDB [(none)]> show create database mydb; # 查看创建数据库mydb时的使用语句
+----------+-----------------------------------------------------------------+
| Database | Create Database                                                 |
+----------+-----------------------------------------------------------------+
| mydb     | CREATE DATABASE `mydb`  |
+----------+-----------------------------------------------------------------+
1 row in set (0.00 sec)

MariaDB [(none)]> alter database mydb character set 'utf8'; # 修改字符集
Query OK, 1 row affected (0.00 sec)

MariaDB [(none)]> show create database mydb; # 查看库创建
+----------+---------------------------------------------------------------+
| Database | Create Database                                               |
+----------+---------------------------------------------------------------+
| mydb     | CREATE DATABASE `mydb`  |
+----------+---------------------------------------------------------------+
1 row in set (0.00 sec)

3.表管理

  1）表创建

★语法：

• CREATE TABLE [IF NOT EXISTS] tbl_name  (create_definition,...) [table_options]

☉create_definition：由逗号分隔的列表

◆字段定义：

• column_name column_defination 字段名称+字段定义相关信息

◆约束定义：

• PRIMARY KEY(col1[,col2, ....])

• UNIQUE KEY 

• FOREIGN KEY 

• CHECK(expr)

◆索引定义：

• {INDEX|KEY}  普通索引创建

• {FULLTEXT|SPATIAL} 全文索引，空间索引

注意：column_definition：

• data_type [NOT NULL | NULL] [DEFAULT default_value] [AUTO_INCREMENT] [UNIQUE [KEY] | [PRIMARY] KEY] [COMMENT 'string']

☉table_option:

• ENGINE [=] engine_name 存储引擎

★查看数据库支持的存储引擎种类：

• mysql> SHOW ENGINES;

★查看指定表的存储引擎：

• mysql> SHOW TABLE STATUS LIKE clause;
  

★查看表结构定义：

• DESC tbl_name；
  

★查看表状态属性信息：

• SHOW TABLE STATUS [{FROM | IN} db_name] [LIKE 'pattern' | WHERE expr]

演示：

  1.表创建：

[root@Centos7 ~]# mysql -p134296
Welcome to the MariaDB monitor.  Commands end with ; or \g.
Your MariaDB connection id is 28
Server version: 5.5.44-MariaDB MariaDB Server

Copyright (c) 2000, 2015, oracle, MariaDB Corporation Ab and others.

Type 'help;' or '\h' for help. Type '\c' to clear the current input statement.

MariaDB [(none)]> SHOW DATABASES;
+--------------------+
| Database           |
+--------------------+
| information_schema |
| mydb               |
| mysql              |
| performance_schema |
| test               |
| ultrax             |
+--------------------+
6 rows in set (0.00 sec)

MariaDB [(none)]> USE mydb;
Database changed
MariaDB [mydb]> CREATE TABLE tbl1 (id SMALLINT UNSIGNED NOT NULL PRIMARY KEY AUTO_INCREMENT,name CHAR(30) NOT NULL,age TINYINT UNSIGNED,gender ENUM('F','M') DEFAULT 'M',UNIQUE KEY(name,gender),INDEX(name));
Query OK, 0 rows affected (0.04 sec)

MariaDB [mydb]> DESC tbl1;
+--------+----------------------+------+-----+---------+----------------+
| Field  | Type                 | Null | Key | Default | Extra          |
+--------+----------------------+------+-----+---------+----------------+
| id     | smallint(5) unsigned | NO   | PRI | NULL    | auto_increment |
| name   | char(30)             | NO   | MUL | NULL    |                |
| age    | tinyint(3) unsigned  | YES  |     | NULL    |                |
| gender | enum('F','M')        | YES  |     | M       |                |
+--------+----------------------+------+-----+---------+----------------+
4 rows in set (0.00 sec)

  2.查看存储引擎类型：

MariaDB [(none)]> show engines;
+--------------------+---------+----------------------------------------------------------------------------+--------------+------+------------+
| Engine             | Support | Comment                                                                    | Transactions | XA   | Savepoints |
+--------------------+---------+----------------------------------------------------------------------------+--------------+------+------------+
| InnoDB             | DEFAULT | Percona-XtraDB, Supports transactions, row-level locking, and foreign keys | YES          | YES  | YES        |
| CSV                | YES     | CSV storage engine                                                         | NO           | NO   | NO         |
| MRG_MYISAM         | YES     | Collection of identical MyISAM tables                                      | NO           | NO   | NO         |
| BLACKHOLE          | YES     | /dev/null storage engine (anything you write to it disappears)             | NO           | NO   | NO         |
| MEMORY             | YES     | Hash based, stored in memory, useful for temporary tables                  | NO           | NO   | NO         |
| PERFORMANCE_SCHEMA | YES     | Performance Schema                                                         | NO           | NO   | NO         |
| ARCHive            | YES     | Archive storage engine                                                     | NO           | NO   | NO         |
| MyISAM             | YES     | MyISAM storage engine                                                      | NO           | NO   | NO         |
| FEDERATED          | YES     | FederatedX pluggable storage engine                                        | YES          | NO   | YES        |
| Aria               | YES     | Crash-safe tables with MyISAM heritage                                     | NO           | NO   | NO         |
+--------------------+---------+----------------------------------------------------------------------------+--------------+------+------------+
10 rows in set (0.00 sec)

 3.查看表状态信息：

MariaDB [mydb]> show table status\G
*************************** 1. row ***************************
           Name: tbl1
         Engine: InnoDB
        Version: 10
     Row_format: Compact
           Rows: 0
 Avg_row_length: 0
    Data_length: 16384
Max_data_length: 0
   Index_length: 32768
      Data_free: 0
 Auto_increment: 1
    Create_time: 2016-10-16 17:54:32
    Update_time: NULL
     Check_time: NULL
      Collation: utf8_general_ci
       Checksum: NULL
 Create_options: 
        Comment: 

# 如果有多个表的话，可以使用where name 或者like 匹配相关的表        
MariaDB [(none)]> use mysql;
Reading table information for completion of table and column names
You can turn off this feature to get a quicker startup with -A

Database changed
MariaDB [mysql]> show tables;
+---------------------------+
| Tables_in_mysql           |
+---------------------------+
| columns_priv              |
| db                        |
| event                     |
| func                      |
| general_log               |
| help_category             |
| help_keyword              |
| help_relation             |
| help_topic                |
| host                      |
| ndb_binlog_index          |
| plugin                    |
| proc                      |
| procs_priv                |
| proxies_priv              |
| servers                   |
| slow_log                  |
| tables_priv               |
| time_zone                 |
| time_zone_leap_second     |
| time_zone_name            |
| time_zone_transition      |
| time_zone_transition_type |
| user                      |
+---------------------------+
24 rows in set (0.02 sec)

MariaDB [mysql]> show table status like 'proc%'\G # 匹配proc相关的表
*************************** 1. row ***************************
           Name: proc
         Engine: MyISAM
        Version: 10
     Row_format: Dynamic
           Rows: 0
 Avg_row_length: 0
    Data_length: 292
Max_data_length: 281474976710655
   Index_length: 4096
      Data_free: 292
 Auto_increment: NULL
    Create_time: 2016-10-12 20:06:15
    Update_time: 2016-10-12 20:06:15
     Check_time: NULL
      Collation: utf8_general_ci
       Checksum: NULL
 Create_options: 
        Comment: Stored Procedures
*************************** 2. row ***************************
           Name: procs_priv
         Engine: MyISAM
        Version: 10
     Row_format: Fixed
           Rows: 0
 Avg_row_length: 0
    Data_length: 0
Max_data_length: 239253730204057599
   Index_length: 4096
      Data_free: 0
 Auto_increment: NULL
    Create_time: 2016-10-12 20:06:15
    Update_time: 2016-10-12 20:06:15
     Check_time: NULL
      Collation: utf8_bin
       Checksum: NULL
 Create_options: 
        Comment: Procedure privileges
2 rows in set (0.01 sec)

--------------------------------------------------------------------------------------------------------------------------------------

  2）表修改

★语法：

• ALTER  TABLE tbl_name [alter_specification [, alter_specification] ...]

☉alter_specification

◆表选项 

• ENGINE=engine_name

     ...

◆表定义 

   字段

• ADD ：增

• DRO：删

• CHANGE ：大改

• MODIFY ：局部范围小改动

键和索引

• ADD {PRIMARY|UNIQUE|FOREIGN} key (col1, col2, ...)

• ADD INDEX(col1, col2, ...)

• DROP {PRIMARY|UNIQUE|FOREIGN} KEY key_name；

• DROP INDEX index_name；

★查看表上的索引信息：

• SHOW INDEXES FROM tbl_name；

命令演示：

MariaDB [mydb]> use mydb
MariaDB [mydb]> show index from tbl1;
+-------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
| Table | Non_unique | Key_name | Seq_in_index | Column_name | Collation | Cardinality | Sub_part | Packed | Null | Index_type | Comment | Index_comment |
+-------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
| tbl1  |          0 | PRIMARY  |            1 | id          | A         |           0 |     NULL | NULL   |      | BTREE      |         |               |
| tbl1  |          0 | name     |            1 | name        | A         |           0 |     NULL | NULL   |      | BTREE      |         |               |
| tbl1  |          0 | name     |            2 | gender      | A         |           0 |     NULL | NULL   | YES  | BTREE      |         |               |
| tbl1  |          1 | name_2   |            1 | name        | A         |           0 |     NULL | NULL   |      | BTREE      |         |               |
+-------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
4 rows in set (0.00 sec)

MariaDB [mydb]> alter table tbl1 drop index name_2; # 删除索引name_2
Query OK, 0 rows affected (0.02 sec)
Records: 0  Duplicates: 0  Warnings: 0

MariaDB [mydb]>  show index from tbl1;
+-------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
| Table | Non_unique | Key_name | Seq_in_index | Column_name | Collation | Cardinality | Sub_part | Packed | Null | Index_type | Comment | Index_comment |
+-------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
| tbl1  |          0 | PRIMARY  |            1 | id          | A         |           0 |     NULL | NULL   |      | BTREE      |         |               |
| tbl1  |          0 | name     |            1 | name        | A         |           0 |     NULL | NULL   |      | BTREE      |         |               |
| tbl1  |          0 | name     |            2 | gender      | A         |           0 |     NULL | NULL   | YES  | BTREE      |         |               |
+-------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
3 rows in set (0.00 sec)

MariaDB [mydb]> desc tbl1;
+--------+----------------------+------+-----+---------+----------------+
| Field  | Type                 | Null | Key | Default | Extra          |
+--------+----------------------+------+-----+---------+----------------+
| id     | smallint(5) unsigned | NO   | PRI | NULL    | auto_increment |
| name   | char(30)             | NO   | MUL | NULL    |                |
| age    | tinyint(3) unsigned  | YES  |     | NULL    |                |
| gender | enum('F','M')        | YES  |     | M       |                |
+--------+----------------------+------+-----+---------+----------------+
4 rows in set (0.01 sec)

MariaDB [mydb]> alter table tbl1 add ClassID TINYINT UNSIGNED NOT NULL; # 新增加一个字段
Query OK, 0 rows affected (0.08 sec)               
Records: 0  Duplicates: 0  Warnings: 0

MariaDB [mydb]> desc tbl1;
+---------+----------------------+------+-----+---------+----------------+
| Field   | Type                 | Null | Key | Default | Extra          |
+---------+----------------------+------+-----+---------+----------------+
| id      | smallint(5) unsigned | NO   | PRI | NULL    | auto_increment |
| name    | char(30)             | NO   | MUL | NULL    |                |
| age     | tinyint(3) unsigned  | YES  |     | NULL    |                |
| gender  | enum('F','M')        | YES  |     | M       |                |
| ClassID | tinyint(3) unsigned  | NO   |     | NULL    |                |
+---------+----------------------+------+-----+---------+----------------+
5 rows in set (0.00 sec)

# 使用modify局部修改放到age的行后面
MariaDB [mydb]> alter table tbl1 modify  ClassID TINYINT UNSIGNED NOT NULL after age;
Query OK, 0 rows affected (0.11 sec)               
Records: 0  Duplicates: 0  Warnings: 0

MariaDB [mydb]> desc tbl1;
+---------+----------------------+------+-----+---------+----------------+
| Field   | Type                 | Null | Key | Default | Extra          |
+---------+----------------------+------+-----+---------+----------------+
| id      | smallint(5) unsigned | NO   | PRI | NULL    | auto_increment |
| name    | char(30)             | NO   | MUL | NULL    |                |
| age     | tinyint(3) unsigned  | YES  |     | NULL    |                |
| ClassID | tinyint(3) unsigned  | NO   |     | NULL    |                |
| gender  | enum('F','M')        | YES  |     | M       |                |
+---------+----------------------+------+-----+---------+----------------+
5 rows in set (0.00 sec)

-------------------------------------------------------------------------------

 3）表删除和查看表创建

★表删除

• DROP  TABLE [IF EXISTS] tbl_name [, tbl_name] ...

• 可以一次删除多个表

★查看表创建语句：

• SHOW CREATE TABLE tbl_name 

 4.索引管理

★引入索引的作用：

• MySQL索引的建立对于MySQL的高效运行是很重要的，索引可以大大提高MySQL的检索速度。

• 实际上，索引也是一张表，该表保存了主键与索引字段，并指向实体表的记录。

★索引类型：

• 聚集索引、非聚集索引：索引是否与数据存在一起；

• 主键索引、辅助索引

• 稠密索引、稀疏索引：是否索引了每一个数据项；

• BTREE（B+）、HASH、R Tree、FULLTEXT

     BTREE：左前缀；

★创建

☉语法：

• CREATE  [UNIQUE|FULLTEXT|SPATIAL] INDEX index_name ON tbl_name (index_col_name,...) 索引字段名称

• index_col_name：

       col_name [(length)] [ASC | DESC]

• {INDEX|KEY}  ：普通索引创建

• {FULLTEXT|SPATIAL} ：全文索引，空间索引

★删除：

• DROP  INDEX index_name ON tbl_name

★查看：

• SHOW {INDEX | INDEXES | KEYS} {FROM | IN} tbl_name  [{FROM | IN} db_name] [WHERE expr]

★使用ALTER 命令添加和删除索引

• ALTER TABLE tbl_name ADD PRIMARY KEY (column_list): 

      该语句添加一个主键，这意味着索引值必须是唯一的，且不能为NULL。

• ALTER TABLE tbl_name ADD UNIQUE index_name (column_list):

      这条语句创建索引的值必须是唯一的（除了NULL外，NULL可能会出现多次）。

• ALTER TABLE tbl_name ADD INDEX index_name (column_list):

      添加普通索引，索引值可出现多次。

• ALTER TABLE tbl_name ADD FULLTEXT index_name (column_list):

      该语句指定了索引为 FULLTEXT ，用于全文索引。

命令演示：

MariaDB [mydb]>  show index from tbl1; # 查看索引
+-------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
| Table | Non_unique | Key_name | Seq_in_index | Column_name | Collation | Cardinality | Sub_part | Packed | Null | Index_type | Comment | Index_comment |
+-------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
| tbl1  |          0 | PRIMARY  |            1 | id          | A         |           0 |     NULL | NULL   |      | BTREE      |         |               |
| tbl1  |          0 | name     |            1 | name        | A         |           0 |     NULL | NULL   |      | BTREE      |         |               |
| tbl1  |          0 | name     |            2 | gender      | A         |           0 |     NULL | NULL   | YES  | BTREE      |         |               |
+-------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
3 rows in set (0.03 sec)

MariaDB [mydb]> drop index name on tbl1; # 删除索引
Query OK, 0 rows affected (0.04 sec)
Records: 0  Duplicates: 0  Warnings: 0

MariaDB [mydb]>  show index from tbl1;
+-------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
| Table | Non_unique | Key_name | Seq_in_index | Column_name | Collation | Cardinality | Sub_part | Packed | Null | Index_type | Comment | Index_comment |
+-------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
| tbl1  |          0 | PRIMARY  |            1 | id          | A         |           0 |     NULL | NULL   |      | BTREE      |         |               |
+-------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
1 row in set (0.00 sec)

MariaDB [mydb]> create index name_and_gender on tbl1(name(5),gender); # 创建索引
Query OK, 0 rows affected (0.04 sec)
Records: 0  Duplicates: 0  Warnings: 0

MariaDB [mydb]>  show index from tbl1; # 查看如下
+-------+------------+-----------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
| Table | Non_unique | Key_name        | Seq_in_index | Column_name | Collation | Cardinality | Sub_part | Packed | Null | Index_type | Comment | Index_comment |
+-------+------------+-----------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
| tbl1  |          0 | PRIMARY         |            1 | id          | A         |           0 |     NULL | NULL   |      | BTREE      |         |               |
| tbl1  |          1 | name_and_gender |            1 | name        | A         |           0 |        5 | NULL   |      | BTREE      |         |               |
| tbl1  |          1 | name_and_gender |            2 | gender      | A         |           0 |     NULL | NULL   | YES  | BTREE      |         |               |
+-------+------------+-----------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
3 rows in set (0.00 sec)

MariaDB [mydb]> show index from tbl1 where Key_name like 'name%'; # 查看指定的索引
+-------+------------+-----------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
| Table | Non_unique | Key_name        | Seq_in_index | Column_name | Collation | Cardinality | Sub_part | Packed | Null | Index_type | Comment | Index_comment |
+-------+------------+-----------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
| tbl1  |          1 | name_and_gender |            1 | name        | A         |           0 |        5 | NULL   |      | BTREE      |         |               |
| tbl1  |          1 | name_and_gender |            2 | gender      | A         |           0 |     NULL | NULL   | YES  | BTREE      |         |               |
+-------+------------+-----------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
2 rows in set (0.04 sec)

 5.VIEW 视图

★虚表：存储下来的SELECT语句；

☉创建：

• CREATE  VIEW view_name [(column_list)] AS select_statement

☉修改：

• ALTER  VIEW view_name [(column_list)] AS select_statement
  

☉删除：

• DROP VIEW [IF EXISTS] view_name [, view_name] ...
  

演示：

MariaDB [testdb]> create table tbl2 (id INT UNSIGNED,name VARCHAR(50),age TINYINT UNSIGNED);
Query OK, 0 rows affected (0.04 sec)

MariaDB [testdb]> insert into tbl2 VALUES (1,'tom',21),(2,'tao',15),(3,'jing',22);
Query OK, 3 rows affected (0.00 sec)
Records: 3  Duplicates: 0  Warnings: 0

MariaDB [testdb]> select * from tbl2;
+------+------+------+
| id   | name | age  |
+------+------+------+
|    1 | tom  |   21 |
|    2 | tao  |   15 |
|    3 | jing |   22 |
+------+------+------+
3 rows in set (0.00 sec)

MariaDB [testdb]> CREATE VIEW testview AS SELECT id,name FROM tbl2; # 创建VIEW
Query OK, 0 rows affected (0.02 sec)

MariaDB [testdb]> SHOW TABLES;  # 查看发现view也作为了一个表；
+------------------+
| Tables_in_testdb |
+------------------+
| tbl1             |
| tbl2             |
| testview         |
+------------------+
3 rows in set (0.00 sec)

MariaDB [testdb]> DESC testview;  # 但是数据只有id和name段，和原表tbl2不同
+-------+------------------+------+-----+---------+-------+
| Field | Type             | Null | Key | Default | Extra |
+-------+------------------+------+-----+---------+-------+
| id    | int(10) unsigned | YES  |     | NULL    |       |
| name  | varchar(50)      | YES  |     | NULL    |       |
+-------+------------------+------+-----+---------+-------+
2 rows in set (0.01 sec)

MariaDB [testdb]> select * from testview;
+------+------+
| id   | name |
+------+------+
|    1 | tom  |
|    2 | tao  |
|    3 | jing |
+------+------+
3 rows in set (0.00 sec)

MariaDB [testdb]> show table status\G   # 查看表的类型，可以看到第3张表为view类型的
*************************** 1. row ***************************
           Name: tbl1
         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: 0
 Auto_increment: NULL
    Create_time: 2016-11-24 15:41:24
    Update_time: NULL
     Check_time: NULL
      Collation: latin1_swedish_ci
       Checksum: NULL
 Create_options: 
        Comment: 
*************************** 2. row ***************************
           Name: tbl2
         Engine: InnoDB
        Version: 10
     Row_format: Compact
           Rows: 3
 Avg_row_length: 5461
    Data_length: 16384
Max_data_length: 0
   Index_length: 0
      Data_free: 0
 Auto_increment: NULL
    Create_time: 2016-11-24 16:43:04
    Update_time: NULL
     Check_time: NULL
      Collation: latin1_swedish_ci
       Checksum: NULL
 Create_options: 
        Comment: 
*************************** 3. row ***************************
           Name: testview
         Engine: NULL
        Version: NULL
     Row_format: NULL
           Rows: NULL
 Avg_row_length: NULL
    Data_length: NULL
Max_data_length: NULL
   Index_length: NULL
      Data_free: NULL
 Auto_increment: NULL
    Create_time: NULL
    Update_time: NULL
     Check_time: NULL
      Collation: NULL
       Checksum: NULL
 Create_options: NULL
        Comment: VIEW
3 rows in set (0.00 sec)

MariaDB [testdb]>  DROP VIEW testview;  # 删除view
Query OK, 0 rows affected (0.00 sec)

MariaDB [testdb]> show tables;
+------------------+
| Tables_in_testdb |
+------------------+
| tbl1             |
| tbl2             |
+------------------+
2 rows in set (0.00 sec)