创建并使用数据库mydb6_product 。

mysql> create database mydb6_product;
Query OK, 1 row affected (0.01 sec)mysql> use mydb6_product;
Database changed

新建employees表。 对于gender，有默认值意味着不为空，在建表时可以选择不写not null；

mysql> create table employees(-> id int primary key,-> name varchar(50) not null,-> age int,-> gender varchar(10) default 'unknown',-> salary float);
Query OK, 0 rows affected (0.03 sec)mysql> desc employees;
+--------+-------------+------+-----+---------+-------+
| Field  | Type        | Null | Key | Default | Extra |
+--------+-------------+------+-----+---------+-------+
| id     | int         | NO   | PRI | NULL    |       |
| name   | varchar(50) | NO   |     | NULL    |       |
| age    | int         | YES  |     | NULL    |       |
| gender | varchar(10) | YES  |     | unknown |       |
| salary | float       | YES  |     | NULL    |       |
+--------+-------------+------+-----+---------+-------+
5 rows in set (0.00 sec)

新建orders表。

mysql> create table orders(-> id int primary key,-> name varchar(100) not null,-> price float,-> quantity int,-> category varchar(50));
Query OK, 0 rows affected (0.02 sec)mysql> desc orders;
+----------+--------------+------+-----+---------+-------+
| Field    | Type         | Null | Key | Default | Extra |
+----------+--------------+------+-----+---------+-------+
| id       | int          | NO   | PRI | NULL    |       |
| name     | varchar(100) | NO   |     | NULL    |       |
| price    | float        | YES  |     | NULL    |       |
| quantity | int          | YES  |     | NULL    |       |
| category | varchar(50)  | YES  |     | NULL    |       |
+----------+--------------+------+-----+---------+-------+
5 rows in set (0.00 sec)

新建invoices表。

auto_increment：当对应的字段，不给值，会自动的被系统触发，系统会从当前字段中已经有的最大值+1操作，得到一个新的不同的值。

外键约束主要定义在从表上,主表则必须是有主键约束或唯一键约束.当定义外键后,要求外键列数据必须在主表的主键列存在或为null。格式为foreign key (从表的字段名称) references 主表名字(主表的字段名称)

mysql> create table invoices(-> number int primary key auto_increment,-> order_id int,-> in_date date,-> total_amount float check(total_amount>0),-> foreign key(order_id) references orders(id));
Query OK, 0 rows affected (0.03 sec)mysql> desc invoices;
+--------------+-------+------+-----+---------+----------------+
| Field        | Type  | Null | Key | Default | Extra          |
+--------------+-------+------+-----+---------+----------------+
| number       | int   | NO   | PRI | NULL    | auto_increment |
| order_id     | int   | YES  | MUL | NULL    |                |
| in_date      | date  | YES  |     | NULL    |                |
| total_amount | float | YES  |     | NULL    |                |
+--------------+-------+------+-----+---------+----------------+
4 rows in set (0.01 sec)