Product information of order.

Column	Type	Constraint	Description
shop	TEXT	PRIMARY KEY	Shop ID. (shop.username)
order_id	INTEGER		Order ID. (order.id)
index	INTEGER		Index of products in added order. 0-origin.
product_id	INTEGER	NOT NULL	Product ID. Product that has same id may be different one when it was ordered. (product.id)
price	INTEGER	NOT NULL	Unit price at when it was ordered. Tax excluded.
quantity	INTEGER	NOT NULL	Quantity. (>= 1.)

CREATE TABLE order_product(
shop        TEXT,
order_id    INTEGER,
index       INTEGER,
product_id  INTEGER  NOT NULL,
price       INTEGER  NOT NULL,
quantity    INTEGER  NOT NULL,
PRIMARY KEY(shop, order_id, index)
);