[SQL] 주문량이 많은 아이스크림들 조회하기

https://school.programmers.co.kr/learn/courses/30/lessons/133027

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with a as (
select
    flavor
    , sum(total_order) total_order
from FIRST_HALF
group by 1
)
, b as (
select
    flavor
    , sum(total_order) total_order
from JULY
group by 1
)
, c as (
select
    a.flavor
    , (a.total_order + b.total_order) as total_order
from a
left join b on a.flavor = b.flavor
)
, base as (
select
    flavor, total_order
from c
union
select
    flavor, total_order
from b
where flavor not in (select distinct flavor from a)
)
select
    flavor
from base
order by total_order desc
limit 3