如何设计一个安全的MySQL表结构来实现身份验证功能？

CREATE TABLE users (
  id INT AUTO_INCREMENT PRIMARY KEY,
  username VARCHAR(255) UNIQUE,
  password VARCHAR(255),
  email VARCHAR(255) UNIQUE,
  created_at DATETIME
);

CREATE TABLE sessions (
  id INT AUTO_INCREMENT PRIMARY KEY,
  user_id INT,
  session_id VARCHAR(255),
  expired_at DATETIME,
  FOREIGN KEY (user_id) REFERENCES users(id)
);

import hashlib
import datetime
import random
import string

def authenticate(username, password):
    # 查询用户表，验证用户名和密码的正确性
    query = "SELECT * FROM users WHERE username = %s AND password = %s"
    cursor.execute(query, (username, hashlib.sha256(password.encode()).hexdigest()))
    user = cursor.fetchone()
    if user:
        # 生成新的会话ID
        session_id = ''.join(random.choices(string.ascii_letters + string.digits, k=32))
        
        # 计算会话的过期时间（例如，30分钟后）
        expired_at = datetime.datetime.now() + datetime.timedelta(minutes=30)
        
        # 将会话ID存储到会话表中
        query = "INSERT INTO sessions (user_id, session_id, expired_at) VALUES (%s, %s, %s)"
        cursor.execute(query, (user['id'], session_id, expired_at))
        connection.commit()
        
        return session_id
    else:
        return None