When a grouping key is specified, the data must be grouped by this key before the aggregation can be computed, as illustrated in figure 13.

指定了分组关键词后，必须在完成聚合计算之前，依据这一关键词对数据进行分组，如图 13 所示。

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Attempting to accomplish grouping of elements by detecting first occurrence of a grouping key and using that as a trigger to form a group. Symptoms are use of.

查找第一个分组键并作为组成分组的触发器来对元素分组。

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If more than one column is aggregated using the same grouping key or without a grouping key, then all the aggregations are computed within the same operation with a single pass over the data.

如果利用相同的分组关键字或者没有分组关键字，来聚合多个列，那么，所有的聚合都在单个数据传递中的相同操作中进行。

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