>>>Actually, then I heard from someone, in a discussion on the subject, that the most complicated and largest contraption in use, an ocean liner, didn't go deeper than ten levels.
>>
>>Last statement is interesting. Of course, once you go beyond one level the only approach that makes sense is a self-referencing/recursive one.
>>
>
>Which fits the general rule that in the database business, there are only three quantities: 0, 1 and many.
>

Don't forget null. We just don't know about those.

I can't tell you how many problems I have run into involving nulls. Now at least I am proactive about it but it's an ongoing battle.