query = User.select().where(User.username == 'charlie')
if query.exists():
    # A user named "charlie" exists.
    cool()

subquery = Child.select(Param('1')).where(Child.parent == Parent.id)
parents_with_children = Parent.select().where(
    Clause(SQL('EXISTS'), subquery))

SELECT * FROM parent
WHERE EXISTS (SELECT 1 FROM child
              WHERE child.parent_id = parent.id);