SELECT T1.product_id,T1.shop_id,T1.price,T2.price FROM (SELECT * FROM tablename WHERE date = '2016-1-6') T1,(SELECT * FROM tablename WHERE date= '2016-1-29') T2 WHERE T1.product_id = T2.product_id AND T1.shop_id = T2.shop_id AND T1.price <> T2.price
で、どうですか。差分は「T2.price - T1.price」ですね。