Simple question, not sure what the subquery is doing exactly in this code. More specifically, how it creates the percentage from order_items using a single table and sum(amount_paid) twice.
when 'kale-smoothie' then 'smoothie'
when 'banana-smoothie' then 'smoothie'
when 'orange-juice' then 'drink'
when 'soda' then 'drink'
when 'blt' then 'sandwich'
when 'grilled-cheese' then 'sandwich'
when 'tikka-masala' then 'dinner'
when 'chicken-parm' then 'dinner'
end as category, round(1.0 * sum(amount_paid) /
(select sum(amount_paid) from order_items) * 100, 2) as pct
group by 1
order by 2 desc;