通过用户模型，对数据库进行增删改查操作。

增加：
db.session.add(user)
db.session.commit()

user = User(username = 'xiao777',password = 'xiao777')
db.session.add(user)
db.session.commit()

查询：
User.query.filter(User.username == 'mis1114').first()

user = User.query.filter(User.username == 'xiao777').first()
print(user.username,user.password)
db.session.commit()

修改：
user.password = '111111'
db.session.commit()

user = User.query.filter(User.username == 'xiao777').first()
user.password = '123456'
db.session.commit()

删除：
db.session.delete(user)
db.session.commit()

user = User.query.filter(User.username == 'xiao777').first()
db.session.delete(user)
db.session.commit()