如何在查询中组合AVG，IF和GROUP BY

Question

我有一个带有3个表的关系数据库。第一个，Categories具有蔬菜，肉类，奶酪等商品的类型。第二个，Clients拥有在商店购买的所有客户。最后一个Cart与客户端ID，类别ID和客户在商店中花费的金额有关系。

我需要打印一份报告，使用MySQL了解每个客户在一个类别中花了多少钱。报告看起来应该是

+--------+---------+------+---------+
| Client | Veggies | Meat | Cheeses |
+--------+---------+------+---------+
| Tom    | 1000    | NULL |    1500 |
| John   | NULL    | NULL |    2000 |
| Alice  | 1000    | 1000 |    1000 |
+--------+---------+------+---------+

现在，我正在使用以下查询

SELECT client.name AS Client, 
       category.name AS Category, 
       AVG(cart.subtotal) AS Total 
  FROM cart 
       INNER JOIN category ON category.id = cart.id_category 
       INNER JOIN clients ON clients.id = cart.id_client 
 GROUP BY cart.id_client,
          cart.id_category

但我明白了：

+--------+----------+-------+
| Client | Category | Total |
+--------+----------+-------+
| Tom    | Veggies  |  1000 |
| Tom    | Cheeses  |  1500 |
| John   | Cheeses  |  2000 |
| Alice  | Veggies  |  1000 |
| Alice  | Meat     |  1000 |
| Alice  | Cheeses  |  1000 |
+--------+----------+-------+