We want to identify credit card customers whose spending is dropping sharply. There's a monthly summary table with each customer's spend per month. Customer 101 spent 2000 in January, 1800 in February and 600 in March. How would you compute month-over-month change for each customer in SQL and flag anyone who dropped more than 50%? What if a month is missing in between?

我们想找出消费明显下滑的信用卡客户。有一张月度汇总表，记录每个客户每月的消费额；客户101一月花了2000，二月1800，三月600。你怎么用SQL算每个客户的环比变化，并标出环比跌超50%的客户？中间缺了一个月怎么办？