Django Academy · 课时

aggregate 与 annotate

汇总整个查询集或逐行汇总

aggregate 与 annotate 是 CoddyKit 上的免费 Django Academy 课时。这是第 1 节课，共 4 节。你可以在下方免费阅读本课时的完整内容 — 然后在浏览器中使用内置代码编辑器和全天候 AI 导师进行实践。这是 Django Academy 学习路径的一部分，你的进度在网页和 CoddyKit 应用中同步。 Django Academy 课程共包含 4 节课。

本课时的部分内容尚未翻译，以英文显示。

Two Ways to Summarize

The ORM gives you two summary tools: aggregate collapses a whole queryset into one result, while annotate adds a value to each row.

What aggregate Returns

Calling aggregate ends the query and hands you a plain Python dictionary, not a queryset, so there is nothing left to filter afterward.

from django.db.models import Avg
Book.objects.aggregate(Avg("price"))
# {"price__avg": 24.5}

Naming Your Result

Pass a keyword to aggregate to name the output key yourself, which reads far better than the auto-generated price__avg label.

Book.objects.aggregate(avg_price=Avg("price"))
# {"avg_price": 24.5}

Common Aggregate Functions

You get the usual SQL toolkit: Count, Sum, Avg, Min, and Max. Each one rolls the whole queryset down to a single number.

from django.db.models import Count, Sum
Book.objects.aggregate(total=Count("id"), revenue=Sum("price"))

What annotate Returns

By contrast, annotate returns a queryset where every object carries a new computed attribute, so you can keep filtering and ordering.

Per-Row Counts

Use annotate with a related count to answer per-object questions, like how many books each author has written.

Author.objects.annotate(num_books=Count("book"))
# each author now has .num_books

Group By Happens for You

When you annotate over a relation, Django adds the GROUP BY clause automatically, grouping by the model you started from.

Filter on an Annotation

Because annotate keeps a queryset, you can filter on the new value, for example to find authors with more than five books.

Author.objects.annotate(n=Count("book")).filter(n__gt=5)

Order by an Annotation

You can also order_by a computed value, so ranking your busiest authors is a one-line query.

Author.objects.annotate(n=Count("book")).order_by("-n")

Filter Before You Annotate

Order matters: a filter before annotate narrows which rows get counted, while a filter after it tests the computed result.

Pick the Right Tool

Ask one question: do you want a single summary for the whole set, or one value per object? That choice decides aggregate versus annotate.

Quick Check

You want each author's book count, kept as a queryset you can still order. Which call fits?

Recap

Remember the split: aggregate returns one summary dict and ends the query, while annotate adds a value per row and keeps a queryset you can filter and order. 🎯

常见问题解答

「aggregate 与 annotate」课时是免费的吗？

是的 — 「aggregate 与 annotate」的完整文本可在网页上免费阅读。要进行交互式练习（内置代码编辑器和全天候 AI 导师）并解锁 Django Academy 课程的其余内容，请升级到 CoddyKit PRO。 Django Academy 课程共包含 4 节课。

「aggregate 与 annotate」这节课中我会学到什么？

汇总整个查询集或逐行汇总你通过在浏览器中直接运行的动手代码来练习 Django Academy，全天候 AI 导师会在你学习这节课的过程中回答你的问题。

学习 Django Academy 需要有经验吗？

无需任何先前经验。CoddyKit 上的 Django Academy 课程适合初学者到高级学习者，你可以从这里开始或从头开始，按照自己的节奏学习。这是第 1 节课，共 4 节。

「aggregate 与 annotate」课时需要多长时间？

大多数 CoddyKit 课程大约需要 5–10 分钟。每节课都很精短且互动，所以你能稳步进步，并在网页和应用中从离开的地方继续。

我能在这节 Django Academy 课中编写并运行代码吗？

能。每节 Django Academy 课都包含内置代码编辑器，你可以在浏览器中直接编写并运行真实代码，并获得即时 AI 反馈 — 无需本地设置。

此课程中的所有课时

← 返回 Django Academy