"""
Useful auxiliary data structures for query construction. Not useful outside
the SQL domain.
"""


class EmptyResultSet(Exception):
    pass


class MultiJoin(Exception):
    """
    Used by join construction code to indicate the point at which a
    multi-valued join was attempted (if the caller wants to treat that
    exceptionally).
    """
    def __init__(self, names_pos, path_with_names):
        self.level = names_pos
        # The path travelled, this includes the path to the multijoin.
        self.names_with_path = path_with_names


class Empty(object):
    pass
