如何设计一个安全的MySQL表结构来实现即时通讯功能？

CREATE TABLE users (
  id INT(11) NOT NULL AUTO_INCREMENT PRIMARY KEY,
  username VARCHAR(50) NOT NULL,
  passWord VARCHAR(255) NOT NULL,
  email VARCHAR(100) NOT NULL,
  created_at TIMESTAMP DEFAULT CURRENT_TIMESTAMP
);

ALTER TABLE users
MODIFY COLUMN password VARCHAR(255) NOT NULL;

ALTER TABLE users
ADD CONSTRAINT unique_email UNIQUE (email);

CREATE TABLE messages (
  id INT(11) NOT NULL AUTO_INCREMENT PRIMARY KEY,
  sender_id INT(11) NOT NULL,
  receiver_id INT(11) NOT NULL,
  content TEXT NOT NULL,
  created_at TIMESTAMP DEFAULT CURRENT_TIMESTAMP
);

ALTER TABLE messages
ADD CONSTRAINT fk_sender
FOREIGN KEY (sender_id) REFERENCES users(id)
ON DELETE CASCADE;

ALTER TABLE messages
ADD CONSTRAINT fk_receiver
FOREIGN KEY (receiver_id) REFERENCES users(id)
ON DELETE CASCADE;

CREATE TABLE friendships (
  user1_id INT(11) NOT NULL,
  user2_id INT(11) NOT NULL,
  created_at TIMESTAMP DEFAULT CURRENT_TIMESTAMP,
  PRIMARY KEY (user1_id, user2_id),
  FOREIGN KEY (user1_id) REFERENCES users(id) ON DELETE CASCADE,
  FOREIGN KEY (user2_id) REFERENCES users(id) ON DELETE CASCADE
);