[LeetCode 1158] Market Analysis I

buyer가 2019년에 주문한 건수를 세 보자. 

2019년도 주문 건수를 count로 세고 ifnull로 없는 사람들을 0으로 채웠다. 

select u.user_id as buyer_id , u.join_date, ifnull(o.orders_in_2019,0) as orders_in_2019
from users as u left join 
        (select buyer_id, count(*) as orders_in_2019
        from Orders
        where year(order_date) = 2019
        group by buyer_id ) 
        as o 
    on o.buyer_id = u.user_id ;