Monthly product statistics.

Column	Type	Constraint	Description
shop	TEXT	PRIMARY KEY	Shop ID. (shop.username)
product_id	INTEGER		Product ID. (product.id)
period	DATE		Year and month values of this column represent period of monthly statistics. The statistics collected between the month to the next month.
view	INTEGER	NOT NULL	View count of the product on the month.
sold	INTEGER	NOT NULL	Sold count of the product on the month.

CREATE TABLE product_statistics (
shop       TEXT,
product_id INTEGER,
period     DATE,
view       INTEGER NOT NULL,
sold       INTEGER NOT NULL,
PRIMARY KEY(shop, product_id, period));