day59

聚合查询
关键字：aggregate
from django.db.models import Max,Min,Count,Sum,Avg

分组查询
    关键字：annotate
    1.最简单的规律
        models后面点什么 就是按什么分组  
    
F与Q查询
    from django.db.modles import F,Q
    F  能够帮助你获取到表中字段所对应的数据
    # 书籍的库存数与卖出数
    models.Book.objects.filter(kucun__gt=F('maichu'))
    
    因为filter过滤的条件的都是and连接 
    modesls.Book.objects.filter(Q(title='python'),Q(price=666))
    modesls.Book.objects.filter(Q(title='python')|Q(price=666))
    modesls.Book.objects.filter(~Q(title='python')|Q(price=666))
    
    # Q进阶用法
    q = Q()
    q.connector = 'or'
    q.children.append(('title','python'))
    q.children.append(('title__icontains','python'))
    q.children.append(('price',666))
    models.Book.objects.filter(q)  # 默认还是and关系  

django中如何开启事务
    from django.db import transaction
    try:
        with transaction.atomic():
            # 事务操作
    except BaseException as e:
        print(e)  

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常见字段及参数
    AutoField()
    DateField()
    DateTimeField()
        auto_now
        auto_now_add
    TextField()
    EmailField()          varchar(...)
    BooleanField()        传布尔值  存0/1

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自定义char字段
    class MyCharField(models.Field):

​

        def db_type(self,connection):
            return 'char(%s)'%self.max_length

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orm查询优化
    only
    defer
    select_related
    prefetch_related

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