In how many categories were podcast reviews created in the last six months of 2016? List them.
created in last six months of 2016 refers to created_at BETWEEN '2016-07-01T00:00:00-07:00' and '2016-12-31T23:59:59-07:00'
SELECT COUNT(DISTINCT T1.category) FROM categories AS T1 INNER JOIN reviews AS T2 ON T2.podcast_id = T1.podcast_id WHERE T2.created_at BETWEEN '2016-07-01T00:00:00-07:00' AND '2016-12-31T23:59:59-07:00'