public static string GetTotalProfitByCategory(BookShopContext db)
        {
            var books = db.Categories
                .Select(x => new
                {
                    name = x.Name,
                    profit = x.CategoryBooks.Sum(a => a.Book.Price * a.Book.Copies)
                })
                .OrderByDescending(x => x.profit)
                .ThenBy(x => x.name)
                .ToList();

            return string.Join(Environment.NewLine, books.Select(x => $"{x.name} ${x.profit:f2}"));

        }