实现如下SQL：

select sum('profit'),sum('order_count') from pro where......;

实现方法：
self::where('id', $id)->first(
    array(
        \DB::raw('SUM(profit) as profit'),
        \DB::raw('SUM(order_count) as order_count')
    )
)->toArray();


转载自：http://www.cnblogs.com/xiaozong/p/5949623.html