MySQL基础进阶：汇总数据-ROLLUP运算符

GROUP BY …… WITH ROLLUP 自动汇总型分组（对 SUM 之类的聚合值进行分组后的汇总），若是多字段分组的话汇总也会是多层次的，注意这是MySQL扩展语法，不是SQL标准语法。
WITH ROLLUP前的属性名一定要在SELECT中呈现出来，如果只是GROUP BY就不一定要在SELECT中表现出来。

SELECT
    pm.name AS payment_method,
    SUM(p.amount) AS total
FROM payments p
JOIN payment_methods pm 
    ON p.payment_method = pm.payment_method_id
GROUP BY pm.name WITH ROLLUP