What is the average rating of all the podcasts with reviews created in 2019?
created in 2019 refers to created_at BETWEEN '2019-01-01T00:00:00' and '2019-12-31T23:59:59'; average rating  = Divide (Sum(rating), Count(podcast_id))
SELECT AVG(rating) FROM reviews WHERE created_at BETWEEN '2019-01-01T00:00:00-07:00' AND '2019-12-31T23:59:59-07:00'