73.Python中ORM聚合函數詳解：Count

Count：用來求某個數據的個數。

在如下全部的示例中所採用的模型爲：

from django.db import models


# 定義做者模型
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)


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

    class Meta:
        db_table = 'publisher'


# 定義圖書模型
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'


# 定義預約圖書的模型
class BookOrder(models.Model):
    book = models.ForeignKey('Book', on_delete=models.CASCADE)
    price = models.FloatField()

    class Meta:
        db_table = 'book_order'

1. 好比：求全部圖書的數量，那麼views.py視圖文件中可使用如下代碼：

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.求全部圖書的數量,由於book表中的數據具備惟一性因此能夠經過計算ID的惟一性來計算圖書的數量。
    count = Book.objects.aggregate(book_count=Count('id'))
    print(count)
    # 打印出結果爲：{'book_count': 4}
    return HttpResponse("success !")

2.求同一種書預約的數量

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.求同一種書的預約數量,一樣仍是針對book表，此處能夠是針對bookorder__id，也能夠是bookorder。默認就是針對book__order表中id字段進行操做
    books = Book.objects.annotate(books_count=Count('bookorder'))
    # print(type(books))
    # <class 'django.db.models.query.QuerySet'>
    # 遍歷QuerySet
    for book in books:
        print("%s,%s" % (book.name,book.books_count))
    # 打印出結果：
    # 三國演義,2
    # 水滸傳,2
    # 紅樓夢,2
    # 西遊記,0
# 打印出原生SQL語句
    print(connection.queries)
# 打印出結果：[{'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. 一樣Count()能夠傳遞參數distinct,默認狀況下，distinct=False.能夠經過指定distinct=True，刪除返回值中相同的數據。例如：求被預約的書有多少種書，示例代碼以下：

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


def index(request):
# 6.求被預約的書都有多少種書
# 首先可使用annotate()進行分組，以後對分組的數據進行刪除相同的數據操做。
    book_nums = Book.objects.annotate(book_nums=Count('bookorder', distinct=True))
    print(book_nums)
    return HttpResponse("success !")

打印出結果：

打印出的結果爲QuerySet對象，不容易分辨具體的數據信息。所以咱們能夠對Book模型的__str__(self)方法進行重寫。

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

重寫模型的__str__(self)方法：

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

    class Meta:
        db_table = 'publisher'

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


# 定義圖書模型
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 "(書名：%s,頁數：%s,價格：%s,打折：%s,做者：%s,出版社：%s)" % (self.name, self.pages, self.price, self.rating, self.author, self.publisher)

打印出結果：

<QuerySet [
<Book: (書名：三國演義,頁數：893,價格：129.0,打折：0.8,做者：羅貫中,47,312587329@qq.com,出版社： ->:清華大學出版社)>, 
<Book: (書名：水滸傳,頁數：983,價格：159.0,打折：0.75,做者：施耐庵,57,1924572@qq.com,出版社： ->:吉林大學出版社)>, 
<Book: (書名：紅樓夢,頁數：1543,價格：199.0,打折：0.85,做者：曹雪芹,42,123521472@qq.com,出版社： ->:浙江大學出版社)>, 
<Book: (書名：西遊記,頁數：1003,價格：159.0,打折：0.75,做者：吳承恩,34,193452272@qq.com,出版社： ->:清華大學出版社)>
]>