SELECT 

    customer, 

    category,

    GROUPING(customer) customer_grouping,

    GROUPING(category) category_grouping,

    SUM(sales_amount) 

FROM customer_category_sales

GROUP BY 

    GROUPING SETS(

        (customer,category),

        (customer),

        (category),

        ()

    )

ORDER BY 

    customer, 

    category;