73.Python中ORM聚合函数详解：Count

Count：用来求某个数据的个数。

在如下全部的示例中所采用的模型为：

from django.db import models


# 定义做者模型
class Author(models.Model):
    name = models.CharField(max_length=100, unique=True)
    age = models.IntegerField()
    email = models.EmailField()

    class Meta:
        db_table = 'author'

    def __str__(self):
        return "%s,%s,%s" % (self.name,self.age, self.email)


# 定义出版社模型
class Publisher(models.Model):
    name = models.CharField(max_length=100,unique=True)

    class Meta:
        db_table = 'publisher'


# 定义图书模型
class Book(models.Model):
    name = models.CharField(max_length=100, unique=True)
    pages = models.IntegerField()
    price = models.FloatField()
    rating = models.FloatField()
    author = models.ForeignKey('Author', on_delete=models.CASCADE)
    publisher = models.ForeignKey('Publisher', on_delete=models.CASCADE)

    class Meta:
        db_table = 'book'


# 定义预约图书的模型
class BookOrder(models.Model):
    book = models.ForeignKey('Book', on_delete=models.CASCADE)
    price = models.FloatField()

    class Meta:
        db_table = 'book_order'

1. 好比：求全部图书的数量，那么views.py视图文件中可使用如下代码：

from django.http import HttpResponse
from .models import Author,Publisher,Book,BookOrder
from django.db.models import Avg,Count,Sum
from django.db import connection


def index(request):
# 4.求全部图书的数量,由于book表中的数据具备惟一性因此能够经过计算ID的惟一性来计算图书的数量。
    count = Book.objects.aggregate(book_count=Count('id'))
    print(count)
    # 打印出结果为：{'book_count': 4}
    return HttpResponse("success !")

2.求同一种书预约的数量

from django.http import HttpResponse
from .models import Author,Publisher,Book,BookOrder
from django.db.models import Avg,Count,Sum
from django.db import connection


def index(request):
# 5.求同一种书的预约数量,一样仍是针对book表，此处能够是针对bookorder__id，也能够是bookorder。默认就是针对book__order表中id字段进行操做
    books = Book.objects.annotate(books_count=Count('bookorder'))
    # print(type(books))
    # <class 'django.db.models.query.QuerySet'>
    # 遍历QuerySet
    for book in books:
        print("%s,%s" % (book.name,book.books_count))
    # 打印出结果：
    # 三国演义,2
    # 水浒传,2
    # 红楼梦,2
    # 西游记,0
# 打印出原生SQL语句
    print(connection.queries)
# 打印出结果：[{'sql': 'SELECT @@SQL_AUTO_IS_NULL', 'time': '0.000'}, {'sql': 'SET SESSION TRANSACTION ISOLATION LEVEL READ COMMITTED', 'time': '0.000'}, {'sql': 'SELECT `book`.`id`, `book`.`name`, `book`.`pages`, `book`.`price`, `book`.`rating`, `book`.`author_id`, `book`.`publisher_id`, COUNT(`book_order`.`id`) AS `books_count` FROM `book` LEFT OUTER JOIN `book_order` ON (`book`.`id` = `book_order`.`book_id`) GROUP BY `book`.`id` ORDER BY NULL', 'time': '0.000'}]
    return HttpResponse("success !")

3. 一样Count()能够传递参数distinct,默认状况下，distinct=False.能够经过指定distinct=True，删除返回值中相同的数据。例如：求被预约的书有多少种书，示例代码以下：

from django.http import HttpResponse
from .models import Book
from django.db.models import Count


def index(request):
# 6.求被预约的书都有多少种书
# 首先可使用annotate()进行分组，以后对分组的数据进行删除相同的数据操做。
    book_nums = Book.objects.annotate(book_nums=Count('bookorder', distinct=True))
    print(book_nums)
    return HttpResponse("success !")

打印出结果：

打印出的结果为QuerySet对象，不容易分辨具体的数据信息。所以咱们能够对Book模型的__str__(self)方法进行重写。

<QuerySet [<Book: Book object (1)>, 
<Book: Book object (2)>, 
<Book: Book object (3)>, 
<Book: Book object (4)>]>

重写模型的__str__(self)方法：

# 定义出版社模型
class Publisher(models.Model):
    name = models.CharField(max_length=100,unique=True)

    class Meta:
        db_table = 'publisher'

    def __str__(self):
        return " ->:%s" % self.name


# 定义图书模型
class Book(models.Model):
    name = models.CharField(max_length=100, unique=True)
    pages = models.IntegerField()
    price = models.FloatField()
    rating = models.FloatField()
    author = models.ForeignKey('Author', on_delete=models.CASCADE)
    publisher = models.ForeignKey('Publisher', on_delete=models.CASCADE)

    class Meta:
        db_table = 'book'

    def __str__(self):
        return "(书名：%s,页数：%s,价格：%s,打折：%s,做者：%s,出版社：%s)" % (self.name, self.pages, self.price, self.rating, self.author, self.publisher)

打印出结果：

<QuerySet [
<Book: (书名：三国演义,页数：893,价格：129.0,打折：0.8,做者：罗贯中,47,312587329@qq.com,出版社： ->:清华大学出版社)>, 
<Book: (书名：水浒传,页数：983,价格：159.0,打折：0.75,做者：施耐庵,57,1924572@qq.com,出版社： ->:吉林大学出版社)>, 
<Book: (书名：红楼梦,页数：1543,价格：199.0,打折：0.85,做者：曹雪芹,42,123521472@qq.com,出版社： ->:浙江大学出版社)>, 
<Book: (书名：西游记,页数：1003,价格：159.0,打折：0.75,做者：吴承恩,34,193452272@qq.com,出版社： ->:清华大学出版社)>
]>