What is the average number of reviews of all the root beer brands from "CA" State?
average = DIVIDE(COUNT(CustomerID), COUNT(BrandID) WHERE state = CA); 

SELECT CAST(COUNT(*) AS REAL) / COUNT(DISTINCT T1.BrandID) AS avgreview FROM rootbeerbrand AS T1 INNER JOIN rootbeerreview AS T2 ON T1.BrandID = T2.BrandID WHERE T1.State = 'CA'