问题描述
Django的 QuerySet 有两种方法， annotate 和 aggregate 。文档说：
Django's QuerySet has two methods, annotate and aggregate. The documentation says that:
它们之间有什么区别吗？如果没有，那么为什么聚合存在？
Is there any other difference between them? If not, then why does aggregate exist?
推荐答案
在示例查询上，而不是从文档中引用。 Aggregate 计算整个查询集的值。 注释在查询集中计算每项的汇总值。
I would focus on the example queries rather than your quote from the documentation. Aggregate calculates values for the entire queryset. Annotate calculates summary values for each item in the queryset.
>>> Book.objects.aggregate(average_price=Avg('price'))
{'average_price': 34.35}
返回包含查询器中所有图书的平均价格的字典。
Returns a dictionary containing the average price of all books in the queryset.
>>> q = Book.objects.annotate(num_authors=Count('authors'))
>>> q[0].num_authors
2
>>> q[1].num_authors
1
q 是图书的查询，但每本书都已经注明了作者的数量。
q is the queryset of books, but each book has been annotated with the number of authors.
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