-- Pobieranie autorów wraz z ilością książek:
SELECT a.first_name, a.last_name, COUNT(DISTINCT b.id) AS writed_books 
FROM public.authors a 
JOIN public.books b ON b.author_id = a.id
GROUP BY a.id

-- Pobieranie książek wraz z ilością recenzji:
SELECT b.title, COUNT(DISTINCT r.id) AS reviews_count, AVG(r.grade) AS avg_grade
FROM public.books b
JOIN public.reviews r ON r.book_id = b.id
GROUP BY b.id

-- Pobranie danych za pomoca widoku:
SELECT * FROM public.average_grades