四十三：数据库之SQLAlchemy之group_by和having子句

group_by：根据某个字段进行分组，比如想要根据年龄进行分组，再统计每一组有多少人
having：对查找结果进一步过滤，类似于SQL语句的where

准备工作

from sqlalchemy import create_engine, Column, Integer, String, Float, Text, ForeignKey, DateTime, Enum
from sqlalchemy.ext.declarative import declarative_base
from sqlalchemy.orm import sessionmaker, relationship, backref

# 数据库信息
host = ‘127.0.0.1‘
port = ‘3306‘
database = ‘db_to_sqlalchemy‘
username = ‘root‘
password = ‘123456‘

# 数据库类型 连接数据库的插件，这里使用的pymysql
DB_URI = f‘mysql pymysql://{username}:{password}@{host}:{port}/{database}‘

engine = create_engine(DB_URI)  # 创建引擎
Base = declarative_base(engine)  # 使用declarative_base创建基类
session = sessionmaker(engine)()


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(String(20), default=‘male‘)


Base.metadata.drop_all()  # 删除所有表
Base.metadata.create_all()  # 创建表

user1 = User(username=‘user1‘, age=20, gender=‘male‘)
user2 = User(username=‘user2‘, age=21, gender=‘female‘)
user3 = User(username=‘user3‘, age=20, gender=‘secret‘)
user4 = User(username=‘user4‘, age=26, gender=‘male‘)
user5 = User(username=‘user5‘, age=19, gender=‘female‘)
session.add_all([user1, user2, user3, user4, user5])
session.commit()