我正在使用postgresql，并尝试在cmd（py list.py）终端中运行以下代码，但无法运行，它显示：
错误消息：

File "list.py", line 3, in <module>
    from sqlalchemy import create_engine
ModuleNotFoundError: No module named 'sqlalchemy'

我需要帮助，因为我试图在虚拟环境目录（pip install sqlalchemy）中运行这个，然后仍然显示相同的错误消息！
代码：

import os

from sqlalchemy import create_engine
from sqlalchemy.orm import scoped_session, sessionmaker

engine = create_engine(os.getenv("DATABASE_URL"))
db = scoped_session(sessionmaker(bind=engine))

def main():
    flights = db.execute("SELECT origin, destination, duration FROM flights").fetchall()
    for flight in flights:
        print(f"{flight.origin} to {flight.destination}, {flight.duration} minutes.")

if __name__ == "__main__":
    main()