c# - 再帰カテゴリと内部結合を使用

Question

再帰的なカテゴリを作成したい.結合を使用している間はアイテムをロードしない.カテゴリにアイテムがない場合は表示されないため、外部結合したくない.どうすればこの問題を解決できますか?

public string CategoryTree(int depth)
{
    StringBuilder sBuilder;   
    using (DataContext db = new DataContext())
    {
        var query = from n in db.Categories join m in db.Products on n.CatId equals m.CategoryId where n.ParentId == depth select new{n, m};
        sBuilder = new StringBuilder("<ul>");
        foreach (var q in query)
        {
            sBuilder.Append("<li>");
            sBuilder.Append("<a href='/Product/"+q.m.ProdId+">'"+q.n.Title+"</a>"+CategoryTree(q.n.CatId));
            sBuilder.Append("</li>");
        }
        sBuilder.Append("</ul>");
    }
    return sBuilder.ToString();
}

Categories
+---------+---------------+------------+
| CatId   | Title         | ParentId   |
+---------+---------------+------------+
|    1    | Electronic    |   NULL     |
+---------+---------------+------------+
|    2    | Computer      |   NULL     |
+---------+---------------+------------+
|    3    | Television    |     1      |
+---------+---------------+------------+
|    4    | Led           |     3      |
+---------+---------------+------------+
|    5    | Printer       |     2      |
+---------+---------------+------------+
|    6    | Laser Printer |     5      |
+---------+---------------+------------+

Products
+---------+---------------+------------+
| ProdId  | ProductName   | CategoryId |
+---------+---------------+------------+
|    1    | Samsung LED   |      4     |
+---------+---------------+------------+
|    2    | Sony LED TV   |      4     |
+---------+---------------+------------+