hierarchy select question?

Dear all,

I would like to define threads in message system for replies to message but if
I define too many level, I am afraid I have problem in the select...

Say, I have define 3 levels:

1
/ \
2 3
/\ \
4 5 6

It means message 2 is a reply to 1.
4 is a further follow-up of 2...etc

In table format, I would present it with

Table A
ParentId ChildId
1 2
1 3
2 4
2 5
3 6

I think I can issue the command to join table A 3 times to give the following
result

1st 2nd 3rd
Row1 1 2 4
Row2 1 2 5
Row3 1 3 6

But can I show
a. which level each node belongs
b. no. of child nodes it has
altogether in one "select" sql

and if the level exceeds 3, how can I do it?

Many thanks.....

Best regards,
Boris