)分组聚合（GroupBy and Aggregation）分组聚合可以在如下的一个MapReduce中完成。Mapper抽取数据并将之分组聚合，Reducer 中对收到的数据再次聚合。
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Using SQL group by and aggregation functions on that table is usually more efficient than producing the same result in plain XQuery.
对该表使用SQL的group by函数和聚集函数通常比使用生成相同结果的普通XQuery更有效。
Those objects are similar to the AggregateGroup used in the first section, and they keep track of the aggregation information for each group that is processed by the UDF.
这些对象与第一节中使用的AggregateGroup类似，也跟踪记录udf所处理的每一个分组的聚集信息。
This paper considers SQL Group-By and aggregation based on merge-sort in parallel database systems.
本文考虑在并行数据库系统中基于归并排序的分组操作和集函数计算。
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