We want to look at retention for newly opened cards. One table has each customer's account open date, and another has every transaction with the customer and date. For example, 1,000 customers opened cards in January, and 420 of them transacted in the following month. How would you write SQL to compute the next-month active rate for each monthly open cohort?

我们想看新开卡客户的留存。一张客户表记录每人的开卡日期，一张交易表记录每笔交易的客户和日期。比如一月新开卡1000人，其中420人在开卡后的第二个月有刷卡。你会怎么用SQL算出每个开卡月份队列的次月活跃率？