What is an aggregation annotation?

In ORM frameworks such as Django or SQLAlchemy, aggregation annotation allows adding calculated fields to query results based on aggregate functions (SUM, COUNT, AVG, MAX, MIN) for related records. It enables obtaining summary information without executing separate subqueries or complex joins manually.

Example in Django ORM:

from django.db.models import Count

# Get a list of authors and the number of books for each
Author.objects.annotate(book_count=Count('book'))

Example in SQLAlchemy ORM:

from sqlalchemy import func

# Get a list of authors and the number of books for each
session.query(Author, func.count(Book.id).label('book_count')).join(Book).group_by(Author).all()

Main advantages:

• Efficiency: Reduces the number of database queries.
• Readability: Makes the code more understandable and concise.
• Flexibility: Allows creating complex summary reports.