While mapping an author element to an author instance isn't too bad, there's not grouping element, like authors, that makes it clear that each author belongs to a single collection in a Book instance.

尽管把一个author元素映射到一个author实例没什么问题，但是没有分组元素，比如authors，它清楚地显示出所有作者属于Book实例中的一个集合。

youdao

The relevant factor that impacts the complexity of the job is the number of different grouping keys, not the number of different aggregated columns that you would see in counting functions.

影响任务复杂性的相关要素是不同分组关键字的数量，而不是在计数函数中所看到的，不同的所聚合列的数量。

youdao

While the ordering of the grouping columns does not matter for a single grouping set or multiple grouping sets, the order of the columns does matter for ROLLUP.

对于单一分组集或多个分组集，分组列的次序是无所谓的；但是对于ROLLUP，列的次序不能随意安排。

youdao

The Tea Party movement is not a political party, but a loose grouping of conservative grass roots activists who oppose the health care bill and taxes and who want to reduce the power of the federal government.

VOA: standard.2010.03.28

And they could not understand "why grouping of payments in bands for one, but not the other, presents data-protection or breach-of-confidence issues", as the BBC had insisted.

BBC: RELATED BBC SITES

Mr Ecevit promptly replied that Mr Dervis had promised not to join the new grouping.

ECONOMIST: A government on the brink | The

The result, it appears: Myanmar will not be admitted to the grouping in 1997.

CNN: No, Not Yet, Myanmar