def select_one(src, rel, dst):
    """
    Create an SQL query that selects one ID from a
    relation table given a source and destination node.

    :param src: The source node.
    :param rel: The relation.
    :param dst: The destination node.
    """
    smt = 'SELECT src FROM %s WHERE src = ? AND dst = ? LIMIT 1'
    return smt % rel, (src, dst)