数据查询 | Django开发

        1.获取所有对象
            object1 = 表名.objects.all()
            # bookinfo = BookInfo.objects.all()
            # 可以通过切片选择范围内对象(限制查询集合)
                object2 = 表名.objects.all()[1,3]
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        2.过滤器(查询集的方法)
            2.1 条件查询
                1.filter() # 满足查询，始终返回一个查询集，列表
                2.exclude() # 不满足查询
                3.order_by()
                4.values()  
                5.all()
            2.2 语法规则：
                # 表名.objects.filter(字段名__运算符判断=‘值‘)
            2.3 运算符
                1.exact  # 精确匹配
                2.contains  # 包含
                3.startswith  # 以指定值开头
                4.endswith  # 以指定值结束
                5.isnull  # 不为空
                6.in  # 范围内查询
                7.gt/gte/lt/lte
                # 运算符前加上i，不区分大小写；
            2.4 例子 
                # filter()过滤器
                # 精确查询
                bookinfo_3 = BookInfo.objects.filter(id__exact=1)
                print(bookinfo_3)

                # 包含查询
                bookinfo4 = BookInfo.objects.filter(book_title__contains=‘八‘)
                print(bookinfo4)

                # 首字查询
                bookinfo5 = BookInfo.objects.filter(book_title__startswith=‘天‘)
                print(bookinfo5)

                # 尾字查询
                bookinfo6 = BookInfo.objects.filter(book_title__endswith=‘传‘)
                print(bookinfo6)

                # 查看不为空的字段值
                bookinfo7 = BookInfo.objects.filter(book_title__isnull=False)
                print(bookinfo7)

                # 查询指定编号范围的字段值
                bookinfo8 = BookInfo.objects.filter(pk__in=[1,3])
                print(bookinfo8)

                # 查询大小范围内的字段值
                bookinfo9 = BookInfo.objects.filter(id__gt=3)
                print(bookinfo9)

                # exclude()过滤器
                bookinfo10 = BookInfo.objects.exclude(id__gt=3)
                print(bookinfo10)
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        3.返回单个值的方法
            3.1 get 获取单一对象
                bookinfo11 = BookInfo.objects.get(book_title = ‘天龙八部‘)
                print(bookinfo11)
                # get如果查询的不仅一个对象，会触发MultipleObjectsReturned异常
                bookinfo12 = BookInfo.objects.get(id__gt=2)
                print(bookinfo12)
                # get如果查询不到对象，会触发DoesNotExist异常
                bookinfo13 = BookInfo.objects.get(book_title = ‘天龙七部‘)
                print(bookinfo13)
            3.2 count()
            3.3 first()
            3.4 last()
            3.5 exists()
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        4.F对象和Q对象
            # 需要导入模块中的对象
            # from django.db.models import F, Q

            4.1 F对象
                # 实现两个属性之间的比较
                bookinfo14 = BookInfo.objects.filter(book_read__gte=F(‘book_comment‘))
                print(bookinfo14)
                # 可以在F()对象上使用算数运算
                bookinfo15 = BookInfo.objects.filter(book_read__gte=F(‘book_comment‘)*2)    
                print(bookinfo15)

            4.2 Q对象(query查询)
                # 构造查询对象
                -- 逻辑与：多个过滤器逐个调用
                    bookinfo16 = BookInfo.objects.filter(book_read__lte=40, id__lte=3)
                    print(bookinfo16)
                    bookinfo17 = BookInfo.objects.filter(book_read__lte=40).filter(id__lte=3)
                    print(bookinfo17)
                -- 利用Q对象实现，或与非
                    -- 基本语法
                        Q(属性名__运算符=值)
                        # &表示逻辑与，|表示逻辑或
                        bookinfo18 = BookInfo.objects.filter(Q(book_read__lte=40) & Q(id__lte=3))
                        print(bookinfo18)
                        bookinfo19 = BookInfo.objects.filter(Q(book_read__lte=40) | Q(id__lte=3))
                        print(bookinfo19)
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        5.聚合函数
            # 需要导入模块
            # from django.db.models import *
            # 表名.objects.aggregate(Sum(字段名))
            1.Avg
            2.Count
                # 查询总数的时候一般不使用；
                # BookInfo.objects.count() 替代；
            3.Max
            4.Min
            5.Sum
                bookinfo20 = BookInfo.objects.aggregate(Sum(‘book_read‘))
                print(bookinfo20)
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        6.关联查询
            6.1 一对多,一类
                1.获取单一字段对象
                    object1 = 表名.objects.get(‘条件‘)
                    # book = BookInfo.objects.get(id=1)
                2.获取对应所有所有对象信息
                    object2 = object1.小写表名_set.all()
                    # heroes = book.heroinfo_set.all()
            6.2 多对一，多类
                1.获取单一字段对象
                    object1 = 表名.objects.get(‘条件‘)
                    # hero = HeroInfo.objects.get(id=1)
                2.获取对应对象信息
                    object1.字段属性
                    # hero.hero_book
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        7.自关联
            # 内部的关系字段指向本表的主键，这就是自关联的表结构；
            # 比如省市查询的模型
            7.1 模型类
                class AreaInfo(models.Model):
                    # 地区名称
                    area_name = models.CharField(max_length=30)
                    # 父级地区
                    area_parent = models.ForeignKey(‘self‘, null=True, blank=True)

                    def __str__(self):
                        return self.area_name
            7.2 测试数据
                areas = [("北京", None),
                         ("河北", None),
                         ("海淀区", 1),
                         ("昌平区", 1),
                         ("顺义区", 1),
                         ("房山区", 1),
                         ("朝阳区", 1),
                         ("丰台区", 1),
                         ("石家庄", 2),
                         ("唐山", 2),
                         ("保定", 2),
                         ("邢台", 2),
                         ("邯郸", 2),
                         ("秦皇岛", 2)]
                         
                for area in areas:
                    area_info = AreaInfo()
                    area_info.area_name = area[0]
                    area_info.area_parent_id = area[1]
                    area_info.save()
                    
            7.3 查询
                AreaInfo.objects.filter(area_parent__area_name=None)
                AreaInfo.objects.filter(area_parent__area_name="北京")
                AreaInfo.objects.filter(area_parent__area_name="河北")
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        8.结果集排序
            8.1 order_by
                # 升序排列
                hero_list = HeroInfo.objects.order_by(‘id‘)
                # 降序排列
                hero_list = HeroInfo.objects.order_by(‘-id‘)
            
            8.2 如果有需求，让数据表排序默认为倒序，可以通过元类来实现
                class HeroInfo(models.Model):
                    hero_name = models.CharField(max_length=20)

                    class Meta:
                        ordering = [‘-id‘]

                # 此时查询结果集默认就是根据ID降序排列的结果
                hero_list = HeroInfo.objects.all()
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