MySql

存储引擎
1）innodb
支持数据库高级功能：事务（要么整体成功要么整体失败）外键
2）myisam
1.不支持事务，外键，只有数据存储基本功能
2.效率非常高

删除数据库

drop database if exists db1;

创建表

create table tb1(
 id int primary key auto_increment,
name varchar(20) not null,
num int not null unique,
xid int,
foreign key(xid) references tb2(id)
)engine=innodb charset=utf8;

更改表

-- 增加gender列
alter table tb1 add gender char(1) after name;

-- 设置字段为空

alter table tb1 modify num int null;

-- 设置自增

alter table tb1 modify id int auto_increment;

-- 去掉自增

alter table tb1 modify id int;

-- 删除主键

alter table tb1 drop primary key;

-- 删除外键

alter table tb1 drop foreign key (外键约束);

-- 删除约束

alter table tb1 drop index 约束名;

-- 删除表

drop table if exists tb1;

-- 查看安装数据库后的编码
show variables like 'char%';
-- 运行结果

root@localhost:(none)>show variables like 'char%';
+--------------------------+---------------------------------------------------------+
| Variable_name            | Value                                                   |
+--------------------------+---------------------------------------------------------+
| character_set_client     | utf8                                                    |
| character_set_connection | utf8                                                    |
| character_set_database   | utf8                                                    |
| character_set_filesystem | binary                                                  |
| character_set_results    | utf8                                                    |
| character_set_server     | utf8                                                    |
| character_set_system     | utf8                                                    |
| character_sets_dir       | C:\Program Files\MySQL\MySQL Server 5.5\share\charsets\ |
+--------------------------+---------------------------------------------------------+
8 rows in set (0.00 sec)

-- 查看特定数据库的编码
show create database test;

root@localhost:test>show create database test;
+----------+---------------------------------------------------------------+
| Database | Create Database                                               |
+----------+---------------------------------------------------------------+
| test     | CREATE DATABASE `test` /*!40100 DEFAULT CHARACTER SET utf8 */ |
+----------+---------------------------------------------------------------+
1 row in set (0.00 sec)