sum,group 一起使用 TP3.2

$order_item = M('order_item');
$order_item_list = $order_item->field(['*','sum(num) as num'])->group('sku_id')->order('num desc')->limit(10)->select();
// SELECT *,sum(num) as num FROM `tf_order_item` GROUP BY sku_id ORDER BY num desc LIMIT 10

group的同时，求和。求和之后，根据和的大小排序。然后获取最高的10条数据。

也可以加上where筛选条件

$order_item = M('order_item');
$order_item_list = $order_item->field(['*','sum(num) as num'])->where(['order_id'=>['in','1,2']])->group('sku_id')->order('num desc')->limit(10)->select();
// SELECT *,sum(num) as num FROM `tf_order_item` WHERE `order_id` IN ('1','2') GROUP BY sku_id ORDER BY num desc LIMIT 10

先where筛选，然后group的同时，求和。求和之后，根据和的大小排序。然后获取最高的10条数据。

SELECT select_list　　　 
FROM table_name　　 
[ WHERE search_condition ]　　 
[ GROUP BY group_by_expression ]　　 
[ HAVING search_condition ]　　 
[ ORDER BY order_expression [ ASC | DESC ] ] 
[ LIMIT m,n] 

/**
 * mysql 执行顺序
 * SELECT select_list　　　 
 * FROM table_name　　 
 * [ WHERE search_condition ]　　 
 * [ GROUP BY group_by_expression ]　　 
 * [ HAVING search_condition ]　　 
 * [ ORDER BY order_expression [ ASC | DESC ] ] 
 * [ LIMIT m,n] 
 */
public function index() {
    $order_item = M('order_item');
    $order_item_list = $order_item->field(['*','sum(num) as num'])->where(['order_id'=>['in','1,2']])->group('sku_id')->order('num desc')->limit(10)->select();
    $this->json->ok($order_item_list);
}