root of parent-child table with NOT NULL constraint

Hello!

I've created the following table that (I hope) represents groups that
have a relationship to their parent group:

   CREATE TABLE Groups
   (
      Id INTEGER,
      ParentId INTEGER
         CONSTRAINT ParentId_Constraint NOT NULL REFERENCES Groups(Id),
      CONSTRAINT Groups_PK PRIMARY KEY (Id)
   );

The question is: how do I add the first group? The very first group is
the root group, and doesn't have a parent.

My choices seem to be to either:

- create the table without the constraint, add a group that will be the
  parent of all "root" groups, then add the constraint. The problem with
  this solution seems to be that dump and restores will be problematic.

- remove the NOT NULL constraint altogether, but then I won't get the
  kind of referential integerity I want.

So, kind sirs/madams, what am I missing?

Thank you!

Bill Wadley