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。
以 任意顺序 返回结果表。
查询结果格式如下所示。
示例 1:
输入： 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 | +------+---------+----------+--------+------------+ 输出： +----------+---------+-------------+----------------+--------------------+-----------------------+ | 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 | +----------+---------+-------------+----------------+--------------------+-----------------------+
解题
注意按月分组，不是按照日期
date_format(trans_date,'%Y-%m')可以替换成substr(trans_date,1,7)
count(if(state='approved',1,null))一定要是null，而不能是0，还可以使用sum(if(state='approved',1,0))替换
select date_format(trans_date,'%Y-%m') as month, country, count(amount) as trans_count, count(if(state='approved',1,null)) as approved_count, sum(amount) as trans_total_amount, sum(if(state='approved',amount,0)) as approved_total_amount from Transactions group by month,country
