20 - 每月交易 I

--  考点：日期转换格式
--  date_format(trans_date,'%Y-%m')select date_format(trans_date,'%Y-%m') month,country,count(*) trans_count,sum(if(state='approved',1,0)) approved_count,sum(amount) trans_total_amount,sum(if(state="approved",amount,0)) approved_total_amount
fromTransactions 
group bymonth,country;