以下为句子列表：
英文: 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.
中文: 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。
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