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)