Limits of SQL

Hi.

I am looking for a way to write a SELECT that finds connectivity
components of a graph or at least for one that given two nodes
determines if there is a path between them. It seems that this is not
possible, no matter what graph representation I choose. Which constructs
from set theory are missing in SQL? Set of all subsets is one I am
missing, or can it be done somehow?

Is anybody else thinking about the limits of SQL? As often I am probably
not the first to ask these questions. Any pointers?

Sincerely,
Joachim