What is the best seller root beer brand and what is the average star rating for this root beer?
best seller root beer refers to MAX(COUNT(BrandID)); average star rating = AVG(StarRating);
SELECT T1.BrandID, AVG(T1.StarRating) FROM rootbeerreview AS T1 INNER JOIN `transaction` AS T2 ON T1.CustomerID = T2.CustomerID INNER JOIN rootbeerbrand AS T3 ON T1.BrandID = T3.BrandID GROUP BY T3.BrandID ORDER BY COUNT(T1.BrandID) DESC LIMIT 1