1.獲取所有對象
            object1 = 表名.objects.all()
            # bookinfo = BookInfo.objects.all()
            # 可以通過切片選擇範圍內對象(限制查詢集合)
                object2 = 表名.objects.all()[1,3]
        ================================================================================
        2.過濾器(查詢集的方法)
            
 
2.1 條件查詢
                1.filter() # 滿足查詢，始終返回一個查詢集，列表
                2.exclude() # 不滿足查詢
                3.order_by()
                4.values()  
                5.all()
            2.2 語法規則：
                # 表名.objects.filter(字段名__運算符判斷=‘值‘)
            2.3 運算符
                1.exact  # 精確匹配
                2.contains  #
 
 包含
                3.startswith  # 以指定值開頭
                4.endswith  # 以指定值結束
                5.isnull  # 不為空
                6.in  # 範圍內查詢
                7.gt/gte/lt/lte
                # 運算符前加上i，不區分大小寫；
            2.4 例子 
                # filter()過濾器
                # 精確查詢
                bookinfo_3 = BookInfo.objects.filter(id__exact=1)
                
 
print(bookinfo_3)

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

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

                # 尾字查詢
                bookinfo6 = BookInfo.objects.filter(book_title__endswith=‘傳‘)
                print(bookinfo6)

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

                # 查詢指定編號範圍的字段值
                bookinfo8 = BookInfo.objects.filter(pk__in=[1,3])
                print(bookinfo8)

                # 查詢大小範圍內的字段值
                bookinfo9 = BookInfo.objects.filter(id__gt=3)
                print(bookinfo9)

                # exclude()過濾器
                bookinfo10 = BookInfo.objects.exclude(id__gt=3)
                print(bookinfo10)
        ================================================================================
        3.返回單個值的方法
            3.1 get 獲取單一對象
                bookinfo11 = BookInfo.objects.get(book_title = ‘天龍八部‘)
                print(bookinfo11)
                # get如果查詢的不僅一個對象，會觸發MultipleObjectsReturned異常
                bookinfo12 = BookInfo.objects.get(id__gt=2)
                print(bookinfo12)
                # get如果查詢不到對象，會觸發DoesNotExist異常
                bookinfo13 = BookInfo.objects.get(book_title = ‘天龍七部‘)
                print(bookinfo13)
            3.2 count()
            3.3 first()
            3.4 last()
            3.5 exists()
        ================================================================================
        4.F對象和Q對象
            # 需要導入模塊中的對象
            # from django.db.models import F, Q

            4.1 F對象
                # 實現兩個屬性之間的比較
                bookinfo14 = BookInfo.objects.filter(book_read__gte=F(‘book_comment‘))
                print(bookinfo14)
                # 可以在F()對象上使用算數運算
                bookinfo15 = BookInfo.objects.filter(book_read__gte=F(‘book_comment‘)*2)    
                print(bookinfo15)

            4.2 Q對象(query查詢)
                # 構造查詢對象
                -- 邏輯與：多個過濾器逐個調用
                    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對象實現，或與非
                    -- 基本語法
                        Q(屬性名__運算符=值)
                        # &表示邏輯與，|表示邏輯或
                        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)
        ================================================================================
        5.聚合函數
            # 需要導入模塊
            # from django.db.models import *
            # 表名.objects.aggregate(Sum(字段名))
            1.Avg
            2.Count
                # 查詢總數的時候一般不使用；
                # BookInfo.objects.count() 替代；
            3.Max
            4.Min
            5.Sum
                bookinfo20 = BookInfo.objects.aggregate(Sum(‘book_read‘))
                print(bookinfo20)
        ================================================================================
        6.關聯查詢
            6.1 一對多,一類
                1.獲取單一字段對象
                    object1 = 表名.objects.get(‘條件‘)
                    # book = BookInfo.objects.get(id=1)
                2.獲取對應所有所有對象信息
                    object2 = object1.小寫表名_set.all()
                    # heroes = book.heroinfo_set.all()
            6.2 多對一，多類
                1.獲取單一字段對象
                    object1 = 表名.objects.get(‘條件‘)
                    # hero = HeroInfo.objects.get(id=1)
                2.獲取對應對象信息
                    object1.字段屬性
                    # hero.hero_book
        ================================================================================
        7.自關聯
            # 內部的關系字段指向本表的主鍵，這就是自關聯的表結構；
            # 比如省市查詢的模型
            7.1 模型類
                class AreaInfo(models.Model):
                    # 地區名稱
                    area_name = models.CharField(max_length=30)
                    # 父級地區
                    area_parent = models.ForeignKey(‘self‘, null=True, blank=True)

                    def __str__(self):
                        return self.area_name
            7.2 測試數據
                areas = [("北京", None),
                         ("河北", None),
                         ("海澱區", 1),
                         ("昌平區", 1),
                         ("順義區", 1),
                         ("房山區", 1),
                         ("朝陽區", 1),
                         ("豐臺區", 1),
                         ("石家莊", 2),
                         ("唐山", 2),
                         ("保定", 2),
                         ("邢臺", 2),
                         ("邯鄲", 2),
                         ("秦皇島", 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 查詢
                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")
        ================================================================================
        8.結果集排序
            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，讓數據表排序默認為倒序，可以通過元類來實現
                class HeroInfo(models.Model):
                    hero_name = models.CharField(max_length=20)

                    class Meta:
                        ordering = [‘-id‘]

                # 此時查詢結果集默認就是根據ID降序排列的結果
                hero_list = HeroInfo.objects.all()
        ================================================================================

數據查詢 | Django開發