How many episodes are there in the 9th season of Law and Order? Calculate the average number of casts per season of the said series.
the 9th season refers to season = 9; Law and Order refers to series = 'Law and Order'; cast refers to category = 'Cast'; average number of casts per season = divide(count(person_id), count(episode_id))
SELECT SUM(CASE WHEN T2.season = 9 THEN 1 ELSE 0 END) AS num , CAST(SUM(CASE WHEN T2.season = 9 THEN 1 ELSE 0 END) AS REAL) / COUNT(T1.episode_id) FROM Credit AS T1 INNER JOIN Episode AS T2 ON T1.episode_id = T2.episode_id WHERE T1.category = 'Cast' AND T2.series = 'Law and Order'
CREATE TABLE Episode
(
    episode_id       TEXT
            primary key,
    series           TEXT,
    season           INTEGER,
    episode          INTEGER,
    number_in_series INTEGER,
    title            TEXT,
    summary          TEXT,
    air_date         DATE,
    episode_image    TEXT,
    rating           REAL,
    votes            INTEGER
);
CREATE TABLE Keyword
(
    episode_id TEXT,
    keyword    TEXT,
    primary key (episode_id, keyword),
    foreign key (episode_id) references Episode(episode_id)
);
CREATE TABLE Person
(
    person_id     TEXT
            primary key,
    name          TEXT,
    birthdate     DATE,
    birth_name    TEXT,
    birth_place   TEXT,
    birth_region  TEXT,
    birth_country TEXT,
    height_meters REAL,
    nickname      TEXT
);
CREATE TABLE Award
(
    award_id       INTEGER
            primary key,
    organization   TEXT,
    year           INTEGER,
    award_category TEXT,
    award          TEXT,
    series         TEXT,
    episode_id     TEXT,
    person_id      TEXT,
    role           TEXT,
    result         TEXT,
    foreign key (episode_id) references Episode(episode_id),
    foreign key (person_id) references Person(person_id)
);
CREATE TABLE Credit
(
    episode_id TEXT,
    person_id  TEXT,
    category   TEXT,
    role       TEXT,
    credited   TEXT,
    primary key (episode_id, person_id),
    foreign key (episode_id) references Episode(episode_id),
    foreign key (person_id) references Person(person_id)
);
CREATE TABLE Vote
(
    episode_id TEXT,
    stars      INTEGER,
    votes      INTEGER,
    percent    REAL,
    foreign key (episode_id) references Episode(episode_id)
);
