Marketing wants to send each customer offers for their top two merchant categories by spend. The transaction table records the customer, merchant category and amount. For example, customer 101 spent 800 on dining, 800 on groceries, 300 on gas and 100 on travel. How would you write SQL to get each customer's top two categories, and what happens when there are ties?

营销团队想给每个客户推送其消费最多的前两个商户类别的优惠。交易表记录了客户、商户类别和金额，比如客户101在餐饮花了800、超市800、加油300、旅游100。你怎么写SQL取每人前两类？如果出现并列，结果会怎样？