题解 | 查询出每个品牌在不同月份的总销售额以及购买该品牌商品的用户的平均年龄

select
    o.category_id,
    round(sum(o.order_amount),2) as total_sales,
    count(case when c.customer_gender='男' then 1 else null end) as male_customers,
    count(case when c.customer_gender='女' then 1 else null end) as female_customers
from 
    order_details o 
left join customer_info c on o.order_id=c.order_id
where
    o.order_date between '2024-01-01' and '2024-06-30'
group by
    o.category_id
order by
    o.category_id asc 

这题一开始我写成count(case when c.customer_gender='男' then 1 else 0 end) as male_customers出错，因为0非空，这行也会被统计

要么写成：count(case when c.customer_gender='男' then 1 else null end) as male_customers

要么写成：sum(case when c.customer_gender='男' then 1 else 0 end) as male_customers