Mapping of item/tag and pricing.

Column	Type	Constraint	Description
shop	TEXT	PRIMARY KEY	Shop ID. (shop.username)
pricing_id	INTEGER		pricing の ID.
target	INTEGER		Type of target. 0: item 1: tag
target_id	INTEGER		Target ID. (item.id/tag.id)

CREATE TABLE pricing_map(
shop       TEXT,
pricing_id INTEGER,
target     INTEGER,
target_id  INTEGER,
PRIMARY KEY (shop, pricing_id, target, target_id)
);