Find out which hotel and travel business having the most review? Calculate the standard deviation of the review star for this business.
"Hotel & Travel" is the category_name; most review refers to Max(Count(category_id)); Average star per user = Divide (Sum (review_stars), Count(user_id))
SELECT T2.category_id FROM Business_Categories AS T1 INNER JOIN Categories AS T2 ON T1.category_id = T2.category_id INNER JOIN Reviews AS T3 ON T3.business_id = T1.business_id WHERE T2.category_name = 'Hotels & Travel' GROUP BY T2.category_id ORDER BY COUNT(T2.category_id) DESC LIMIT 1