1. 题目

Table: Transactions

+---------------+---------+
| 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。
state 列类型为 “[”批准“，”拒绝“] 之一。

编写一个 sql 查询来查找每个月和每个国家/地区的事务数及其总金额、已批准的事务数及其总金额。

查询结果格式如下所示：

Transactions table:
+------+---------+----------+--------+------------+
| id   | country | state    | amount | trans_date |
+------+---------+----------+--------+------------+
| 121  | US      | approved | 1000   | 2018-12-18 |
| 122  | US      | declined | 2000   | 2018-12-19 |
| 123  | US      | approved | 2000   | 2019-01-01 |
| 124  | DE      | approved | 2000   | 2019-01-07 |
+------+---------+----------+--------+------------+Result table:
+----------+---------+-------------+----------------+--------------------+-----------------------+
| month    | country | trans_count | approved_count | trans_total_amount | approved_total_amount |
+----------+---------+-------------+----------------+--------------------+-----------------------+
| 2018-12  | US      | 2           | 1              | 3000               | 1000                  |
| 2019-01  | US      | 1           | 1              | 2000               | 2000                  |
| 2019-01  | DE      | 1           | 1              | 2000               | 2000                  |
+----------+---------+-------------+----------------+--------------------+-----------------------+

来源：力扣（LeetCode）
链接：https://leetcode-cn.com/problems/monthly-transactions-i
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 date_format(trans_date, '%Y-%m') month, country,count(*) trans_count,sum(case state when 'approved' then 1else 0 end) approved_count,sum(amount) trans_total_amount,sum(if(state='approved', amount, 0)) approved_total_amount
from Transactions
group by date_format(trans_date, '%Y-%m'), country
# %M 输出的是英文的月份，%m是两位数字
# %Y 输出4为年份, %y 输出年分的后两位

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