尽管可利用自连接在XQuery中表示分组和聚集，但那非常糟糕。
Although grouping and aggregation can be expressed in XQuery using self-joins, it is quite awkward.
最后，我们可以获取和更新特定分组的聚集信息。
Finally, the aggregation information for a certain group can be retrieved and updated.
包含连接、分组、聚集和排序操作的决策支持查询。
Decision-support queries that involve joins, groups, aggregates and sort operations.
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