มีกี่เพลงที่อยู่ในแนวร็อค?
How many tracks belong to rock genre?
df_track = pd.DataFrame(columns=['GenreId']); df_genre = pd.DataFrame(columns=['GenreId', 'Name'])
len(df_track[df_track['GenreId'].isin(df_genre[df_genre['Name'] == 'Rock']['GenreId'])])