第二题
SELECT
product_rate AS 放款笔数,
loan_prin AS 放款金额,
ifnull(SUM((overdue_days>30)*(loan_prin-paid_principal))/SUM(loan_prin),0) AS 30天以上金额逾期率
FROM loan_list
WHERE year(loan_date)=2018 AND MONTH(loan_date)BETWEEN 2 AND 5
GROUP BY product_rate;
逻辑函数可以用IFNULL函数