How many different products are available to order?
select count(distinct product_id) from products
Order them by the number of times they have been ordered.
select * from products as t1 join regular_order_products as t2 on t1.product_id = t2.product_id group by t2.product_id order by count(*)
Which has been ordered the greatest number of times?
select * from products as t1 join regular_order_products as t2 on t1.product_id = t2.product_id group by t2.product_id order by count(*) desc limit 1
Only show the name and price.
select t1.product_name, t1.product_price from products as t1 join regular_order_products as t2 on t1.product_id = t2.product_id group by t2.product_id order by count(*) desc limit 1