通过选择其他表中的另一列插入一列,但如何填充第二列

ALTER TABLE users ADD COLUMN age INT DEFAULT 18;

from sqlalchemy import Column, Integer
from sqlalchemy.ext.declarative import declarative_base
from sqlalchemy.orm import sessionmaker

Base = declarative_base()

class User(Base):
    __tablename__ = 'users'
    id = Column(Integer, primary_key=True)
    name = Column(String)
    age = Column(Integer, default=18)

engine = create_engine('sqlite:///example.db')
Base.metadata.create_all(engine)

Session = sessionmaker(bind=engine)
session = Session()

# 更新现有用户的年龄
users = session.query(User).all()
for user in users:
    user.age = 18
session.commit()

ALTER TABLE users ADD COLUMN age INT DEFAULT 18;