在filter之后的是会在过滤后的结果上进行聚合操作，在annotate之后也是可以进行filter的。
Book.objects.filter(name__startswith="Django").annotate(Count('author'）
Book.objects.filter(name__startswith="Django").aggregate(Avg('price'))
Book.objects.annotate(num_authors=Count('authors')).filter(num_authors__gt=1)
annotation可以作为排序的依据
之前所述，annotation是给匹配到的每行产生一个值
当使用values时，则是会按照给定values字段组合，annotation是对应group的。
group by
注意，values在annotate之前才能完成该操作，否则还是在产生了annotation之后再拿到values。
filter 在filter之后的是会在过滤后的结果上进行聚合操作，在annotate之后也是可以进行filter...
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本文标题：aggregation other things
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