Marketing wants to know how many cardholders had zero transactions in March. The customer table has 1,000 people, and the transaction table records the customer, date and amount of each transaction. A colleague left-joined customers to transactions, filtered for March in the WHERE clause, then counted customers with no matching transaction, and got zero. What went wrong, and how would you fix it?

市场部想知道3月有多少信用卡客户一笔交易都没有。客户表有1000人，交易表记录每笔交易的客户、日期和金额。同事用客户表左连接交易表，在条件里筛出3月的交易，再数交易为空的客户，结果是0。你觉得问题出在哪，怎么改？