--8. Select average price of the cheapest books offered in each category (Find the cheapest books in categories and than find the average for this set). Round the amount to 2 decimal places.

select cast(avg(T.price) as decimal(5,2)) as avg_cost from (select min (o.price) price from yrb_offer o, yrb_book b where o.title = b.title and o.year = b.year group by cat) T
