def select_optional_row(statement, build_arg = nil)
    rows = select(statement, build_arg)
    raise UnexpectedQueryResult, "selected #{rows.size} rows; expected 0 or 1" if rows.size > 1
    if rows.empty? then nil else rows[0] end
  end
def select_optional_row(statement, build_arg = nil)
    rows = select(statement, build_arg)
    raise UnexpectedQueryResult, "selected #{rows.size} rows; expected 0 or 1" if rows.size > 1
    if rows.empty? then nil else rows[0] end
  end
Simple convenience wrapper around the generic select call.
 Throws an exception unless the result set is empty or a single row.
 Returns nil if the result set is empty, or the row selected.