Calculate number of descendants recursively

I have a table with navigation that joins back on its self using ParentId. I am trying to calculate how many descendents each record has, I know that I need to increment a counter in the recursion, I'm just not sure how to go about it!

Any help would be greatly appreciated!

CREATE TABLE [dbo].[Navigation](
    [Id] [int] I开发者_如何学运维DENTITY(1,1) NOT NULL,
    [AnchorText] [varchar](50) NOT NULL,
    [ParentID] [int] NULL)

insert into Navigation
select 'Level 1', null
union
select 'Level 2', 1
union
select 'Level 3', 2
union
select 'Level 4', 3


WITH NavigationCTE (Id, AnchorText, ParentId, NumberofDescendants) as
(
      Select Id, AnchorText, ParentId, 'Number of Descendants Here' as NumberofDescendants
      from dbo.Navigation nav

      union ALL 

      select nav.Id, nav.AnchorText, nav.ParentId,  'Number of Descendants Here' as NumberofDescendants
      from dbo.Navigation nav

      join Navigation ON nav.ParentId = nav.Id
)


SELECT * FROM NavigationCTE

EDIT Added Level and Incremented in recursion:

WITH NavigationCTE (Id, AnchorText, ParentId, Level) as
(
      Select nav.Id, nav.AnchorText, nav.ParentId, 0 as Level
      from dbo.Navigation AS nav

      UNION ALL 

      select nav.Id, nav.AnchorText, nav.ParentId, Level + 1
      from dbo.Navigation AS nav

      join Navigation AS nav2 ON nav.ParentId = nav2.Id
)


SELECT * FROM NavigationCTE

---

Common Table Expressions provide the kind of recursive functionality you need. Study the creation and use of the Level field in the article's sample query. It performs exactly the kind of incrementing you want to achieve, starting with 0 in the anchor query.

Working query based on provided sample:

WITH NavigationCTE  AS
(   
    SELECT navA.[Id], navA.ParentId, 0 AS depth_lvl
    FROM Navigation as navA

    UNION ALL

    SELECT navB.Id, navB.ParentId, depth_lvl + 1
    FROM Navigation AS navB
    JOIN NavigationCTE AS nav_cte_a
        --ON navB.ParentId = nav_cte_a.Id
        ON nav_cte_a.ParentId = navB.Id
)
SELECT Id, ParentId, coalesce(max(depth_lvl),0)
FROM NavigationCTE
GROUP BY Id, ParentId
ORDER BY Id, ParentId