要完成这项任务，应该首先确定这个属性出现在哪个元素上。

To accomplish this, you should begin by figuring out which element the attribute appears on.

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这些属性出现在 SELECT 子句和 GROUP BY 子句中。

The attributes appear in both the SELECT clause and the GROUP BY clause.

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比如，dom树中可能出现默认属性，或者作为验证结果出现新的SAX事件。

For example, default attributes can appear in a DOM tree or new SAX events can occur as a result of validation.

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