If you were using some kind of representation for Nested Sets -- left-to-right depth-first numbering, or a human-readable id.id.id chain -- then it's really easy to write a constraint for that: parent left < myleft, right > myright, or dotted_id.split('.').filter{|first, rest| return false if rest.contains first} (yeah, yeah, that second pseudocode would be unrealistically PITA for some DBs).

More generally:

I'm not a big SQL wonk anymore, but I find a lot of people have the intuition that relational databases are ill-suited for trees.

An intuition that is much closer to the truth is that almost all databases can handle trees pretty well, because there's still an unambiguous concept of ordering and containment, and you can usually arrange things so as to do range/ancestor/inclusion queries efficiently.

It's graphs with loops/without unambiguous concept of ordering/containment that are really hard.