I realize this is actually an SQL question but I haven't been able to figure out the answer.

In a simple self-referential table, the following produces all the Nodes that are parents to some child node(s):

node_table_alias = node_table.alias() parents = session.query(Node).filter(Node.id == node_table_alias.c.parent_id)

I can't figure out the analogous query that produces all the Nodes that are not parents to another node. It is clear that:

non_parents = session.query(Node).filter(Node.id != node_table_alias.c.parent_id)

doesn't work but I can't figure out what the right query is. Any help would be appreciated.