public static string GetBooksByCategory(BookShopContext db, string input)
        {
            StringBuilder sb = new StringBuilder();

            List<string> inputCategories = input.Split(" ", StringSplitOptions.RemoveEmptyEntries).Select(c => c.ToLower()).ToList();

            List<int> categoryIds = new List<int>();

            List<string> books = new List<string>();

            foreach (var category in inputCategories)
            {
                var categoryId = db.Categories.Where(c => c.Name == category)
                    .Select(c => c.CategoryId);

                categoryIds.AddRange(categoryId);
            }

            foreach (var id in categoryIds)
            {
                var book = db.Books
                    .Where(b => b.BookCategories.Any(bc => bc.CategoryId == id))
                    .OrderBy(b => b.Title)
                    .Select(b => b.Title)
                    .ToList();

                books.AddRange(book);
            }

            foreach (var title in books)
            {
                sb.AppendLine($"{title}");
            }

            return sb.ToString().Trim();
        }