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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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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Note that it is easy to get a list of all the grouping-key values (region names in this example) without setting up grouping; just use the new distinct-values function.

要注意，不需要建立分组就能很容易得到所有grouping - key值(该例中为区域名称)的列表，只要使用新的distinct - values函数即可。

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