windows安装MySQL时需要注意：
1.检查本机主机名是否为英文，不能为中文（若不是修改后，重启计算机）
2.若已经安装了mysql，则：
（1）.检查是否安装了历史mysql，要求在系统中卸载
（2）.删除磁盘中所有mysql命名文件夹
（3）.清除注册表
3.关闭所有安全类软件：360全家桶 杀毒（火绒） 管家
4.准备好运行库（可选）
5.找到运行服务界面
6.找到环境变量设置界面（添加MySQL的安装路径）
7.找到命令控制台或powershell （注意管理员权限运行）
安装完成之后若登录不进去需要考虑：密码是否有误、MySQL服务是否启动，终端是否为管理员权限运行的……
创建数据库
mysql> create database mydb6_product;
Query OK, 1 row affected (0.01 sec)#查看数据库
mysql> show databases;
+--------------------+
| Database |
+--------------------+
| information_schema |
| mydb2_stuinfo |
| mydb3_employee |
| mydb4_product |
| mydb5_sales |
| mydb6_product |
| mydb_temp1 |
| mydbl_test |
| mysql |
| performance_schema |
| sys |
| temp1 |
+--------------------+
12 rows in set (0.00 sec)根据题目要求是需要在库中创建三张表，需要查看所新建的库是否为当前使用，若不是则需要选择使用它作为当前库
mysql> select database();
+------------+
| database() |
+------------+
| NULL |
+------------+
1 row in set (0.00 sec)mysql> use mydb6_product
Database changed
mysql> select database();
+---------------+
| database() |
+---------------+
| mydb6_product |
+---------------+
1 row in set (0.00 sec)创建三张表命令如下：
mysql> create table employees(-> id int primary key, -> name varchar(50) not null, -> age int not null, -> gender varchar(10) default "unknown",-> salary float(5,2) not null);
Query OK, 0 rows affected, 1 warning (0.02 sec)mysql> desc employees;
+--------+-------------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+--------+-------------+------+-----+---------+-------+
| id | int | NO | PRI | NULL | |
| name | varchar(50) | NO | | NULL | |
| age | int | NO | | NULL | |
| gender | varchar(10) | YES | | unknown | |
| salary | float(5,2) | NO | | NULL | |
+--------+-------------+------+-----+---------+-------+
5 rows in set (0.00 sec)mysql> create table orders(-> id int primary key,-> name varchar(100) not null,-> price float(5,2) not null,-> quantity int not null,-> category varchar(50) not null);
Query OK, 0 rows affected, 1 warning (0.04 sec)mysql> desc orders;
+----------+--------------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+----------+--------------+------+-----+---------+-------+
| id | int | NO | PRI | NULL | |
| name | varchar(100) | NO | | NULL | |
| price | float(5,2) | NO | | NULL | |
| quantity | int | NO | | NULL | |
| category | varchar(50) | NO | | NULL | |
+----------+--------------+------+-----+---------+-------+
5 rows in set (0.00 sec)mysql> create table invoices(-> number int primary key auto_increment,-> order_id int not null,-> in_date date not null,-> total_amount float(5,2) check(total_amount>0),-> foreign key(order_id) references orders(id));
Query OK, 0 rows affected, 1 warning (0.03 sec)mysql> desc invoices;
+--------------+------------+------+-----+---------+----------------+
| Field | Type | Null | Key | Default | Extra |
+--------------+------------+------+-----+---------+----------------+
| number | int | NO | PRI | NULL | auto_increment |
| order_id | int | NO | MUL | NULL | |
| in_date | date | NO | | NULL | |
| total_amount | float(5,2) | YES | | NULL | |
+--------------+------------+------+-----+---------+----------------+
4 rows in set (0.00 sec)#查看当前数据库中的表，也可以作为检验三张表是否创建完成，是否在指定的数据库中
mysql> show tables;
+-------------------------+
| Tables_in_mydb6_product |
+-------------------------+
| employees |
| invoices |
| orders |
+-------------------------+
3 rows in set (0.00 sec)
