рейтинг для каждой книги - COUNT записей
SELECT b.*, COALESCE(SUM(br.rating), 0) rating
FROM books b
LEFT JOIN books_rating br ON br.book_id = b.id
GROUP BY b.id
SELECT range_id, MIN(sernum), MAX(sernum)
FROM table_name
GROUP BY range_id
SELECT
item_id,
I.price * COALESCE(R.rate, 1) price_in_byr
FROM Items AS I
LEFT JOIN Rates AS R ON
I.currency = R.currency AND
R.date = (SELECT MAX(date) FROM Rates WHERE Rates.currency = I.currency)
WHERE
item_id = 5;