我选择并更新一行数据 sqlalchemy ，当我第二次执行时， sqlalchemy 什么也不还给我，为什么？

db_connection = 'mysql+pymysql://%s:%s@%s/%s?charset=utf8' % (db_user, db_pwd, db_host, db_name)
    engine = create_engine(db_connection, echo=False)
    session = sessionmaker(autocommit=False, autoflush=False, bind=engine)()
    while True:
        row = session.execute('select id, avi_id, address, code from job  where job_status = :job_status', {'job_status': 'created'}).fetchone()
        if row:
            session.execute('update job set job_status = :job_status where id = :job_id', {'job_status': "finished", 'job_id': row[0]})
            session.commit()
            time.sleep(3)

然后我更新 job_status 进入 created 在mysql客户端上：

update job set job_status = "created" where job_status="finished"

第二个循环（第一个代码块），变量行没有，为什么？