mysql计算两个字段的积的总和

$sql1 = "SELECT SUM(goods_amount*goods_user_rate) FROM " . $GLOBALS['ecs']->table('buy_info') . $where2;

goods_amount和good_user_rate是数据库的两个字段，请问正确的写法是怎样？

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select sum(price * num) from table

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SELECT SUM(ret) FROM(
SELECT goods_amount*goods_user_rate as ret FROM xxxx
);
我知道这样傻的写法是可以的。