SQL question

question.sql

CREATE TABLE track(
    id SERIAL PRIMARY KEY,
    name VARCHAR NOT NULL
);

CREATE TABLE playlist(
    id SERIAL PRIMARY KEY,
    name VARCHAR NOT NULL
);

-- Many-to-many relationship:
-- One track can be included in several playlists
-- One playlists contains several tracks
CREATE TABLE track_to_playlist(
    id SERIAL PRIMARY KEY,
    trace_id INTEGER NOT NULL UNIQUE REFERENCES track(id),
    playlist_id INTEGER NOT NULL UNIQUE REFERENCES playlist(id)
);

-- Task: write an SQL query to retrieve all tracks 
--       which are not included into any playlist.