1、准备数据库环境,比之前增加了price字段
class Publisher(models.Model): name = models.CharField(max_length=32) def __str__(self): return '<Publisher: {} - {}>'.format(self.pk, self.name) class Book(models.Model): title = models.CharField(max_length=32) price = models.DecimalField(max_digits=5,decimal_places=2) #最大999.99 publisher = models.ForeignKey('Publisher',related_name='books',related_query_name='book',on_delete=models.CASCADE) def __str__(self): return '<Book: {} - {}>'.format(self.pk, self.title) class Author(models.Model): name = models.CharField(max_length=32) books = models.ManyToManyField('Book') #多对多创建,后面是要关联的表Book类
聚合
import os if __name__ == '__main__': os.environ.setdefault("DJANGO_SETTINGS_MODULE", "about_orm.settings") import django django.setup() from app01 import models from django.db.models import Max,Min,Sum,Avg,Count # ret = models.Book.objects.aggregate(Max('price')) #aggregate本身就是聚合 ret = models.Book.objects.all().aggregate(Min('price'),Avg('price'),max=Max('price')) #aggregate本身就是聚合 # print(ret) #{'price__max': Decimal('99.00'), 'price__min': Decimal('10.00'), 'price__avg': 68.5}
分组
import os if __name__ == '__main__': os.environ.setdefault("DJANGO_SETTINGS_MODULE", "about_orm.settings") import django django.setup() from app01 import models from django.db.models import Max,Min,Sum,Avg,Count # ret = models.Book.objects.aggregate(Max('price')) #aggregate本身就是聚合 ret = models.Book.objects.all().aggregate(Min('price'),Avg('price'),max=Max('price')) #aggregate本身就是聚合 # print(ret) #{'price__max': Decimal('99.00'), 'price__min': Decimal('10.00'), 'price__avg': 68.5} # 按照出版社进行分组,聚合字段不再我这,我用跨表book__price 最后的values是取我的querySet里面的字段和对应的值 ret = models.Publisher.objects.annotate(Avg('book__price')).values() #annotate:注释 把做完的结果塞到对象里面了 # print(ret) #按照书做分组 再以publisher__name做二次分组 # ret = models.Book.objects.values('publisher__name').annotate(avg=Avg('price')) # # print(ret) # for i in ret: # print(i) #统计一本书作者的个数 # ret = models.Book.objects.annotate(author_num=Count('author')) # for i in ret: # print(i.author_num) # 统计出每个出版社买的最便宜的书的价格 # 方法一: # ret = models.Publisher.objects.annotate(Min('book__price')).values() # for i in ret: # print(i) #方法二:按照书分组 指定values,指定以publisher做了分组的字段 # ret = models.Book.objects.values('publisher').annotate(Min('price')) # for i in ret: # print(i) #统计不止一个作者的图书 # ret = models.Book.objects.annotate(count=Count('author')).filter(count__gt=1) # for i in ret: # print(i) #<Book: 2 - 葵花宝典>
我们在这里先复习一下SQL语句的分组。
假设现在有一张公司职员表:
我们使用原生SQL语句,按照部门分组求平均工资:
select dept,AVG(salary) from employee group by dept;
ORM查询:
from django.db.models import Avg Employee.objects.values("dept").annotate(avg=Avg("salary").values("dept", "avg")
连表查询的分组:
SQL查询:
select dept.name,AVG(salary) from employee inner join dept on (employee.dept_id=dept.id) group by dept_id;
ORM查询:
from django.db.models import Avg models.Dept.objects.annotate(avg=Avg("employee__salary")).values("name", "avg")
更多示例:
示例1:统计每一本书的作者个数
>>> book_list = models.Book.objects.all().annotate(author_num=Count("author")) >>> for obj in book_list: ... print(obj.author_num) ... 2 1 1
示例2:统计出每个出版社买的最便宜的书的价格
>>> publisher_list = models.Publisher.objects.annotate(min_price=Min("book__price")) >>> for obj in publisher_list: ... print(obj.min_price) ... 9.90 19.90
方法二:
>>> models.Book.objects.values("publisher__name").annotate(min_price=Min("price")) <QuerySet [{'publisher__name': '沙河出版社', 'min_price': Decimal('9.90')}, {'publisher__name': '人民出版社', 'min_price': Decimal('19.90')}]>
示例3:统计不止一个作者的图书
>>> models.Book.objects.annotate(author_num=Count("author")).filter(author_num__gt=1) <QuerySet [<Book: 番茄物语>]>
示例4:根据一本图书作者数量的多少对查询集 QuerySet进行排序
>>> models.Book.objects.annotate(author_num=Count("author")).order_by("author_num") <QuerySet [<Book: 香蕉物语>, <Book: 橘子物语>, <Book: 番茄物语>]>
示例5:查询各个作者出的书的总价格
>>> models.Author.objects.annotate(sum_price=Sum("book__price")).values("name", "sum_price") <QuerySet [{'name': '小精灵', 'sum_price': Decimal('9.90')}, {'name': '小仙女', 'sum_price': Decimal('29.80')}, {'name': '小魔女', 'sum_price': Decimal('9.90')}]>