year([date_column]) -> date형식이 들어가있는 컬럼의 year만,
month([date_column]) -> date형식이 들어가있는 컬럼의 month만
where절에 like에 사용한다면 쉽게 date형식을 찾을수 있음
date(joined) >= '2021-10-01' 도 가능.
select year(SALES_DATE) YEAR, month(SALES_DATE) MONTH , count(distinct user_id) PUCHASED_USERS,
round(count(distinct user_id) /
(SELECT count(*)
from user_info
where joined LIKE '2021%'), 1) PUCHASED_RATIO
from user_info join online_sale using(user_id)
where joined LIKE '2021%'
group by year, month
order by year, month문제 출처 : https://school.programmers.co.kr/learn/courses/30/lessons/131534
