create table `customer`
(
	id_customer int auto_increment,
	customer_name varchar(255) not null,
	constraint customer_pk
		primary key (id_customer)
);

create table `orders`
(
	id_order int auto_increment,
	order_nr int not null,
	fk_customer int not null,
	constraint order_pk
		primary key (id_order),
	constraint order_customer_id_customer_fk
		foreign key (fk_customer) references customer (id_customer)
);

#customer
insert into `customer`(id_customer, customer_name) 
values (1, 'John Doe'), (2, 'Sarah Doe'), (3, 'Frank Doe');

insert into `orders`(id_order, order_nr, fk_customer) 
values (1, 123456, 1), (2, 123457, 1), (3, 123458, 2);