小编bla*_*vvv的帖子

SELECT SUM(orders.quantity) AS num, fName, surname
FROM author
INNER JOIN book ON author.aID = book.authorID;

我不断收到错误消息:"您试图执行不包含指定表达式"fName"的查询作为聚合函数的一部分.我该怎么办？