select
today.resource_id,
(today.avg_price - yesterday.avg_price) / yesterday.avg_price as percent_increase
from
item_prices today,
item_prices yesterday
where today.resource_id = yesterday_resource_id
and DATE(FROM_UNIXTIME(today.timestamp)) = $today
and DATE(FROM_UNIXTIME(yesterday.timestamp)) = $yesterday
order by percent_increase desc
limit 10
This is a self join; apologies for the archaic join syntax, it's a bad habit I find hard to shake.