通过Group By操作符可以生成一个结果集，其中包括所需的聚合操作。
The Group By operator enables you to generate a result set that includes the required aggregate operations.
聚合、组合之类操作的操作符可以是以成员、属性或常量作为输入的操作符。
Operators for aggregation, combination, and such manipulation are provided by means of operators that could take members, attributes, or constants as inputs.
其中一个操作符(图1中下方的分支)为一个Group By操作符提供输入，后者执行目标表的两个列所需的聚合。
One of these operators (shown on the lower branch in Figure 1) feeds a Group by operator that performs the aggregations required by two of the target table's columns.
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