Marketing gives you two tables: a customer table with one row per customer and their credit limit, and a card table with one row per card, where a customer can have multiple cards. Customer 101 has a 50K limit and two cards; customer 102 has a 30K limit and one card. A colleague joined the two tables on customer and summed the limit, reporting 130K total. Is that right? How would you calculate it, and how would you catch this kind of issue?

市场部给你两张表：客户表每人一行，记录授信额度；卡片表每张卡一行，一个客户可能有多张卡。客户101额度5万、有两张卡，客户102额度3万、一张卡。同事把两表按客户合并后对额度求和，报总授信13万。这个数对吗？你会怎么算，怎么提前发现这种问题？