problem with tree design

[nithinics]

Hi all,
I've to develop a multilevel marketing web app. In this Im facing an issue, please help me in this issue

MemberX
\/
child1 child2 child3 child4 child5 child6 child7
\/
child11 child22 child33 child44 child55 child66 child77

each child can have 7 childs and upto 7 levels.

my table design is as following

tree
-----

id
parent_id
position
level


id | parent_id | position | level
---------------------------------
2 | 1 | 0 | 1
3 | 1 | 1 | 1
4 | 1 | 2 | 1
5 | 1 | 3 | 1
6 | 2 | 1 | 1


My problem is how to find a vacant position for given parent.

eg: when member 2 (id=2) is adding a new member i've to add the new member 7 in to a vacant position
under him or under any of his descendants.
Is there any way find a vacacnt position ? Or any alternate database design for this purpose ?
Pls help...

Thanks & regards

[SpacePhoenix]

Have a read of this SitePoint Article:

http://www.sitepoint.com/hierarchical-data-database/

It discusses two methods of storing hierarchical data in a relational database