orace一分钟粒度的数据如何变为30分钟粒度

假如只有time，quality两个字段，现在的时间是一分钟一条的粒度，想把它变成30分钟粒度的，quality字段将30分钟的数据相加！

    SELECT TO_TIMESAMP(FLOOR(TO_NUMBER(time)*48)/48.0) AS time,
           SUM(quality) AS quality
      FROM table1
  GROUP BY TO_TIMESAMP(FLOOR(TO_NUMBER(time)*48)/48.0) AS time