What product id was ordered most often?
select product_id from order_items group by product_id order by count ( * ) desc limit 1
and the product ID that was ordered least often?
select product_id from order_items group by product_id order by count ( * ) limit 1
How many times was product id 2 ordered?
select count ( * ) from order_items group by product_id having product_id = "2"
How many times was product id 13 ordered?
select count ( * ) from order_items group by product_id having product_id = "13"