1. 题目

Transactions 记录表

+----------------+---------+
| Column Name    | Type    |
+----------------+---------+
| id             | int     |
| country        | varchar |
| state          | enum    |
| amount         | int     |
| trans_date     | date    |
+----------------+---------+
id 是这个表的主键。
该表包含有关传入事务的信息。
状态列是类型为 [approved（已批准）、declined（已拒绝）] 的枚举。

Chargebacks 表

+----------------+---------+
| Column Name    | Type    |
+----------------+---------+
| trans_id       | int     |
| charge_date    | date    |
+----------------+---------+
退单包含有关放置在事务表中的某些事务的传入退单的基本信息。
trans_id 是 transactions 表的 id 列的外键。
每项退单都对应于之前进行的交易，即使未经批准。

编写一个 SQL 查询，以查找每个月和每个国家/地区的已批准交易的数量及其总金额、退单的数量及其总金额。

注意：在您的查询中，给定月份和国家，忽略所有为零的行。

查询结果格式如下所示：

Transactions 表：
+------+---------+----------+--------+------------+
| id   | country | state    | amount | trans_date |
+------+---------+----------+--------+------------+
| 101  | US      | approved | 1000   | 2019-05-18 |
| 102  | US      | declined | 2000   | 2019-05-19 |
| 103  | US      | approved | 3000   | 2019-06-10 |
| 104  | US      | declined | 4000   | 2019-06-13 |
| 105  | US      | approved | 5000   | 2019-06-15 |
+------+---------+----------+--------+------------+Chargebacks 表：
+------------+------------+
| trans_id   | trans_date |
+------------+------------+
| 102        | 2019-05-29 |
| 101        | 2019-06-30 |
| 105        | 2019-09-18 |
+------------+------------+Result 表：
+----------+---------+----------------+-----------------+-------------------+--------------------+
| month    | country | approved_count | approved_amount | chargeback_count  | chargeback_amount  |
+----------+---------+----------------+-----------------+-------------------+--------------------+
| 2019-05  | US      | 1              | 1000            | 1                 | 2000               |
| 2019-06  | US      | 2              | 8000            | 1                 | 1000               |
| 2019-09  | US      | 0              | 0               | 1                 | 5000               |
+----------+---------+----------------+-----------------+-------------------+--------------------+

来源：力扣（LeetCode）
链接：https://leetcode-cn.com/problems/monthly-transactions-ii
著作权归领扣网络所有。商业转载请联系官方授权，非商业转载请注明出处。

2. 解题

# Write your MySQL query statement below
select *
from
(select t.month, t.country, ifnull(sum(t1.approved_count),0) approved_count, ifnull(sum(t1.approved_amount),0) approved_amount,ifnull(sum(t2.chargeback_count),0) chargeback_count, ifnull(sum(t2.chargeback_amount),0) chargeback_amountfrom (select distinct country, date_format(trans_date, '%Y-%m') monthfrom Transactionsunionselect distinct country, date_format(ch.trans_date, '%Y-%m') monthfrom Chargebacks ch left join Transactions tron ch.trans_id = tr.id) tleft join(select date_format(trans_date, '%Y-%m') month, country, count(*) approved_count,ifnull(sum(amount),0) approved_amountfrom Transactionswhere state='approved'group by month, country) t1on t.month = t1.month and t.country = t1.countryleft join (select date_format(ch.trans_date, '%Y-%m') month,country,count(*) chargeback_count,ifnull(sum(amount),0) chargeback_amountfrom Chargebacks ch left join Transactions tron ch.trans_id = tr.idgroup by month, country) t2on t.month = t2.month and t.country = t2.countrygroup by month, country
) tmp
where tmp.approved_count != 0 or tmp.chargeback_count != 0

or 简单写法，创建一个 chargeback state

select date_format(a.trans_date,'%Y-%m') month,country,sum(state = 'approved') approved_count,sum(if(state = 'approved',amount,0)) approved_amount,sum(state = 'chargeback') chargeback_count,sum(if(state = 'chargeback',amount,0)) chargeback_amount  
from
(select * from transactionswhere state = 'approved'union allselect id, country, 'chargeback' state, amount, c.trans_datefrom chargebacks c left join transactions t on c.trans_id = t.id
) a
group by month,country

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