mysql建立主键

CREATE TABLE users (
    user_id INT PRIMARY KEY,
    username VARCHAR(50) NOT NULL,
    email VARCHAR(100) NOT NULL
);

CREATE TABLE orders (
    order_id INT,
    user_id INT,
    order_date DATE,
    PRIMARY KEY (order_id, user_id)
);

CREATE TABLE products (
    product_id INT AUTO_INCREMENT PRIMARY KEY,
    product_name VARCHAR(100) NOT NULL,
    price DECIMAL(10, 2) NOT NULL
);

INSERT INTO users (user_id, username, email) VALUES (1, 'john', 'john@example.com')
ON DUPLICATE KEY UPDATE username='john', email='john@example.com';

CREATE TABLE users (
    user_id INT PRIMARY KEY NOT NULL,
    username VARCHAR(50) NOT NULL,
    email VARCHAR(100) NOT NULL
);

CREATE TABLE users (
    user_id INT,
    username VARCHAR(50) NOT NULL,
    email VARCHAR(100) NOT NULL,
    PRIMARY KEY (user_id, username)
);