leetcode 1158. Market Analysis I

# Write your MySQL query statement below
select user_id as buyer_id, join_date, 
    sum(case when order_id is not null then 1 else 0 end) as orders_in_2019
from users
left join orders on users.user_id = orders.buyer_id 
and year(order_date) = 2019
group by user_id
order by buyer_id