【Flask】Sqlalchemy group_by having

### group_by：
根据某个字段进行分组。比如想要根据性别进行分组，来统计每个分组分别有多少人

### having：

having是对查找结果进一步过滤。比如只想要看未成年人的数量，那么可以首先对年龄进行分组统计人数，然后再对分组进行having过滤。示例代码如下：

 # coding:utf-8
 # Author: liangjun.chen

 from datetime import datetime
 from sqlalchemy import create_engine, Column, Integer, String, Enum, func

 from sqlalchemy.ext.declarative import declarative_base
 from sqlalchemy.orm import sessionmaker, relationship, backref

 HOSTNAME = '127.0.0.1'
 PORT = 3306
 DATABASE = 'sqlalchemy_first'
 USERNAME = 'root'
 PASSWORD = ''

 DB_URI = 'mysql+pymysql://{username}:{password}@{host}:{port}/{dbname}?charset=utf8'.format(
         username=USERNAME, password=PASSWORD, host=HOSTNAME, port=PORT, dbname=DATABASE
 )
 engine = create_engine(DB_URI)
 Base = declarative_base(engine)
 Session = sessionmaker(engine)
 session = Session()

 class User(Base):
     __tablename__ = 'user'
     id = Column(Integer, primary_key=True, autoincrement=True)
     username = Column(String(50), nullable=False)
     age = Column(Integer, default=0)
     gender = Column(Enum('male', 'female', "secret"), default='male')

     def __repr__(self):
         return "User<{}>".format(self.username)
 # Base.metadata.drop_all()
 # Base.metadata.create_all()
 #
 # user = User(username='saber', age=17, gender='male')
 # user2 = User(username='saber2', age=18, gender='male')
 # user3 = User(username='saber3', age=18, gender='female')
 # user4 = User(username='saber4', age=19, gender='female')
 # user5 = User(username='saber5', age=19, gender='female')
 #
 # session.add_all([user, user2, user3, user4, user5])
 # session.commit()

 # group_by

 users = session.query(User.age, func.count(User.id)).group_by(User.age)
 print users.all()

 # having
 users = session.query(User.age, func.count(User.id)).group_by(User.age).having(User.age>18).all()
 print users