We want a monthly customer report, one row per customer, showing total spend and total payments for the month. The transaction table records customer, month and amount for each purchase; the payment table records customer, month and payment amount. In March, customer 101 made three purchases of 100, 200 and 300, and two payments of 250 each. A colleague joined the two tables directly on customer and summed, getting 1,200 in spend and 1,500 in payments. Why, and how would you fix it?

我们想做客户月度报表，每人一行，列出当月消费总额和还款总额。交易表每笔记录客户、月份、金额；还款表每笔记录客户、月份、还款额。客户101在3月刷了100、200、300三笔，还了250、250两笔。同事把两张表按客户直接连起来再求和，消费成了1200、还款成了1500。为什么？怎么改？