How Do I Loop Thru A Record Set In A Stored Procedure?

Below is a stored procedure that designed to populate a drop down menu system on a website. It works fine as long as the 'id's in the first select start at 1 and are sequential. It fails to grab all the sub tables if the ids are not sequential. So, how do I structure the loop so that the WHERE clause uses not the loop iterator, but rather, the ids from the first Select statement.

Alternatively, is there a more elgant approach that will return the same set of recordsets?

Any help would be much appreciated
Thanks

ALTER PROCEDURE dbo.OPA_GetMenuItems
AS
Declare @i tinyint ,
@tc tinyint
Set @i = 1

/* Select for top level menu items*/

SELECT id, label, url, sort
FROM mainNav
ORDER BY sort

Set @tc = @@rowcount

while @i <= @tc

begin
Set @i = (@i + 1)

/* Select for submenu items*/
SELECT id, label, url, sort, mainNavId
FROM SubNav
WHERE (mainNavId = @i)
ORDER BY mainNavId, sort
end

RETURN