我在一个数据库中有四个表。表格如下所示：

Cashier(cashier_id*, cashier_name)
Category(category_id*, category_name)
Product(product_id*, product_name, price, category_id**)
Purchase(product_id**, cashier_id**, amount)

* primary key
**foreign key

我想选择购买次数最多的类别名称。这是我到目前为止所做的，但是我没有得到我想要的结果

SELECT x.category,x.amount
    FROM (SELECT category_name as category, SUM(amount) as amount FROM 
          Category c, Product pr, Purchase pu WHERE pr.product_id = 
          pu.product_id and c.category_id = pr.category_id GROUP BY 
          category_name) x
    GROUP BY x.category
    HAVING MAX(x.amount);