1·You can retrieve the current contents of the shopping cart (with subtotals) at any time by using a SELECT query.
您可以使用一个SELECT查询随时检索购物车的当前内容(包含小计)。
2·If you want to create subtotals or show other summary information for subsets of a table, you create groups using an aggregate query.
如果要创建小计或显示表的子集的其他汇总信息,可使用聚合查询创建组。