mysql 设置主键

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

ALTER TABLE users ADD PRIMARY KEY (id);

CREATE TABLE orders (
    order_id INT,
    product_id INT,
    quantity INT,
    PRIMARY KEY (order_id, product_id)
);

INSERT IGNORE INTO users (id, username, email) VALUES (1, 'user1', 'user1@example.com');

INSERT INTO users (id, username, email) VALUES (2, 'user2', NULL); -- 正确
INSERT INTO users (id, username, email) VALUES (NULL, 'user3', 'user3@example.com'); -- 错误

REPAIR TABLE users;

-- 创建表并设置主键
CREATE TABLE users (
    id INT AUTO_INCREMENT,
    username VARCHAR(50) NOT NULL,
    email VARCHAR(100),
    PRIMARY KEY (id)
);

-- 在已有表上添加主键
ALTER TABLE users ADD PRIMARY KEY (id);