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'
CREATE TABLE runs (
        run_at text not null,
        max_rowid integer not null,
        reviews_added integer not null
    );
CREATE TABLE podcasts (
        podcast_id text primary key,
        itunes_id integer not null,
        slug text not null,
        itunes_url text not null,
        title text not null
    );
CREATE TABLE IF NOT EXISTS "reviews"
(
    podcast_id TEXT    not null
        constraint reviews_podcasts_podcast_id_fk
            references podcasts,
    title      TEXT    not null,
    content    TEXT    not null,
    rating     INTEGER not null,
    author_id  TEXT    not null,
    created_at TEXT    not null
);
CREATE TABLE IF NOT EXISTS "categories"
(
    podcast_id TEXT not null
        constraint categories_podcasts_podcast_id_fk
            references podcasts,
    category   TEXT not null,
    constraint "PRIMARY"
        primary key (podcast_id, category)
);
CREATE INDEX category_podcast_id_idx
    on categories (podcast_id);
