防止在SQL中插入重复信息

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

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

INSERT INTO users (id, email, username, password)
VALUES (1, 'user@example.com', 'user1', 'password')
ON DUPLICATE KEY UPDATE username='user1', password='password';

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