Question: Among the books purchased by less than 1 dollar, what is the difference between the number of books with less than 500 pages and books with greater than 500 pages?
SQL: SELECT SUM(CASE WHEN T1.num_pages < 500 THEN 1 ELSE 0 END) - SUM(CASE WHEN T1.num_pages > 500 THEN 1 ELSE 0 END) AS dif FROM book AS T1 INNER JOIN order_line AS T2 ON T1.book_id = T2.book_id WHERE T2.price < 1