[SQL 고득점 Kit][JOIN][Level 5] 상품을 구매한 회원 비율 구하기

https://school.programmers.co.kr/learn/courses/30/lessons/131534

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select year(sales_date) as year, month(sales_date) as month, 
count(distinct o.user_id) as purchased_users, 
round(count(distinct o.user_id) / c.count, 1) as purchased_ratio
from ONLINE_SALE o join user_info u 
on o.user_id = u.user_id 
join (select count(u.user_id) as count from user_info u where year(u.joined) = 2021) c
where year(u.joined) = 2021
group by year, month
order by year, month