Product information.

Column	Type	Constraint	Description
shop	TEXT	PRIMARY KEY	Shop ID. (shop.username)
id	INTEGER		Product ID. (item.id)
price	INTEGER	NOT NULL	Unit price. Tax excluded.
stock	INTEGER		Stock of product. If the vlaue is NULL, the product does not require the "Stock."
weight	INTEGER		Weight of product (g.) If the value is NULL, the weight is not specified yet.
release	TIMESTAMP WITH TIME ZONE	NOT NULL	Release date.

CREATE TABLE product(
shop     TEXT,
id       INTEGER,
price    INTEGER  NOT NULL,
stock    INTEGER,
weight   INTEGER,
release  TIMESTAMP WITH TIME ZONE,
PRIMARY KEY (shop, id));