【LeetCode刷题】数据库之中等题：1205. 每月交易II

题目：



分析：该题分了两种情况，分别是approved，和 chargebasks的情况，而且出现了两个日期，因此直接采用join是行不通的，只能用合并的方式。结果中的年月用date_format(date,format)实现。

解法：

1. 查出满足state='approved’条件的各类信息。

select country,state,amount,date_format(trans_date,"%Y-%m") as month,1 as tag
from Transactions
where state='approved'

2. 再查出回退单子(chargebasks)的信息，此处需要right join chargebasks表,然后跟上面的union all合并。

select country,state,amount,date_format(c.trans_date,"%Y-%m") as month,0 as tag
from Transactions t right join Chargebacks c
on c.trans_id=t.id

3. 最后用count，sum根据条件聚合，再根据country,month分组。

▲这里使用tag是为了更好的区分两种不同情况

解：

select month,
country,
count(case when state='approved' and tag=1 then 1 end) as approved_count,
sum(case when tag=1 then amount else 0 end) as approved_amount,
count(case when tag=0 then 1 end) as chargeback_count,
sum(case when tag=0 then amount else 0 end) as chargeback_amount
from(
    select country,state,amount,date_format(trans_date,"%Y-%m") as month,1 as tag
from Transactions
where state='approved'
union all
select country,state,amount,date_format(c.trans_date,"%Y-%m") as month,0 as tag
from Transactions t right join Chargebacks c
on c.trans_id=t.id
) as a
group by month,country;