Django ORM多表查询

以下是Django ORM多表查询的几个例子：

1. 跨表查询

from app01 import models

# 查询所有出版社及其旗下的书籍
publishers = models.Publisher.objects.all()
for publisher in publishers:
    books = publisher.book_set.all()
    print(publisher.name, [book.title for book in books])

2. join查询

from app01 import models

# 查询所有出版社及其旗下的书籍，使用join查询
publishers = models.Publisher.objects.select_related().all()
for publisher in publishers:
    books = publisher.book_set.all()
    print(publisher.name, [book.title for book in books])

3. 聚合查询

from django.db.models import Count, Avg, Sum, Max, Min
from app01 import models

# 查询每个出版社旗下的书籍数量
publishers = models.Publisher.objects.annotate(book_count=Count('book'))
for publisher in publishers:
    print(publisher.name, publisher.book_count)

4. 分组查询

from django.db.models import Count, Avg, Sum, Max, Min
from app01 import models

# 查询每个作者出版的书籍数量
authors = models.Author.objects.annotate(book_count=Count('book'))
for author in authors:
    print(author.name, author.book_count)