这条简单的MySQL查询语句为什么会报错?(如图)

Question

如图：

Answer 1

你要么

SELECT order_num FROM orderitems;

要么

SELECT SUM(quantity *item_price) FROM orderitems;

再要么

SELECT order_num, SUM(quantity*item_price) FROM orderitems GROUP BY order_num;

现在是怎么回事？你期望返回几行，每行的order_num是什么？

错误信息也很明确了，SUM是聚合函数，而order_num是非聚合列。