递归类别和内部联接

Question

I want to make recursive category menu for product categories on my web page.Each category must retrieve related first item according to CategoryId on "Product" table but this category should be disappear if category hasn't any product.Actually, i can make to easily with using INNER JOIN for non-recursive category menu.How can i solve this issue?Is there any idea?

I can use a method as follow but this method both amateur and may be null first item.

Category table

+--------------+---------------+------------+
|  CategoryId  | CategoryName  | ParentId   |
+--------------+---------------+------------+
|      1       | Cookware      |   NULL     |
+--------------+---------------+------------+
|      2       | Tableware     |   NULL     |
+--------------+---------------+------------+
|      3       | Teapots       |     1      |
+--------------+---------------+------------+
|      4       | Cutleries     |     3      |
+--------------+---------------+------------+
|      5       | 30pcs Cutlery |     2      |
+--------------+---------------+------------+

Product table

+--------------+--------------+--------------------+------------+
|  ProductId   | ProductCode  | ProductName        | CategoryId |
+--------------+--------------+--------------------+------------+
|       1      |   G110090    |   Teapot           |      3     |
+--------------+--------------+--------------------+------------+
|       2      |   D220623    |   Cutlery Set      |      5     |
+--------------+--------------+--------------------+------------+ 

RecursiveCategory method

public string RecursiveCategory(IEnumerable<Category> category, int? parent)
{
    string catNode = string.Empty;
    if(category.Any(n=>n.ParentId == parent))
    {
        catNode += "<ul>";
            foreach(Category c in category)
            {
                catNode += "<li>";
                catNode += "<a href='/Detail/" + GetFirstItem(c.CategoryId).ProductId + "'>"+c.CategoryName+"</a>";
                catNode += RecursiveCategory(category, c.ParentId);
                catNode += "</li>";
            }
        catNode += "</ul>"
    }
    return catNode;
}

GetFirstItem method

public Product GetFirstItem(int categoryId)
{
    Product prod = new Product();
    foreach(Product p in db.Product.Where(n=>n.CategoryId == categoryId))
    {
        prod.ProductId = p.ProductId;
        prod.ProductName = p.ProductName;
        ...
    }
    return prod;
}

Answer 1

Try this to construct a hierarchy from a given point (using null in the first call will give you the full tree with the products for each category). As a modification you could make product lazy load if you need to:

public class Category
{
  IEnumerable<Category> Children {get;set;}
  IEnumerable<Product> Products {get;set;}
}

public IEnumerable<Category> GetCategory(int? parent)
{
  var result = new List<Category>();
  foreach (var cat in categories.Where(p => p.parentId = parent)
  {
     var generatedCategory = new Category();
     generatedCategory.Children = GetCategory(cat.id);
     generatedCategory.Products = products.Where(p => p.CategoryId = cat.CategoryId);
     result.Add(generatedCategory);
  }
  return result;
}

Note: I haven't tested the code just a guide about how to construct it easily.