众所周知，GROUP BY会对结果集进行排序。进而消耗较大的资源开销。 在MySQL 5.7版本中，以下哪些写法能避免文件排序？ A select col1,sum(col2) from table group by col1 desc; B select col1,sum(col2) from table group by col1 order by 1=1; C select col1,sum(col2) from table group by col1 order by 1; D select col1,sum(col2) from table group by col1 asc; E select col1,sum(col2) from table group by col1 order by rand(); F select col1,sum(col2) from table group by col1 order by null; 答案是bf