The reason is that the results are unpredictable unless you always group by a primary or candidate key value because you cannot know which value from the non - grouped data will be selected 这个原因是因为结果是不可预知的，除非你允许以主关键字或是候选关键字来作为分组，因为你无法知道which将从哪能一个非分组的数据作选取。
