So, I'm building my hibernate queries dynamically with Criteria (that I love so much).
Then I come accross a problem while trying to do a not:

Criteria criteria = session.createCriteria(MyValueObj.class);
criteria.add(Expression.isNotNull("dateCreated"));
criteria.add(Expression.not(Expression.eq("deleteCode", "D")));

This yields no results, suck.
It looks like it generating some bad SQL.

I decide to try refactoring the statements into:

Criteria criteria = session.createCriteria(MyValueObj.class);
criteria.add(
Expression.not(
Expression.and(
Expression.isNull("dateCreated"),
Expression.eq("deleteCode", "D"))));

And all is well and happy in the world. Perhaps there is some kind of bug in the implementation of the not expression. Investigation will have to wait until another time.
When all else fails, find another way to skin the cat.