def exon_intervals(self):
        """List of (start,end) tuples for each exon of this transcript,
        in the order specified by the 'exon_number' column of the
        exon table.
        """
        results = self.db.query(
            select_column_names=["exon_number", "start", "end"],
            filter_column="transcript_id",
            filter_value=self.id,
            feature="exon")
        sorted_intervals = [None] * len(results)
        for (exon_number, start, end) in results:
            sorted_intervals[int(exon_number) - 1] = (start, end)
        return sorted_intervals
List of (start,end) tuples for each exon of this transcript,
        in the order specified by the 'exon_number' column of the
        exon table.