把mysql字段设置为非空

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

ALTER TABLE users
MODIFY COLUMN username VARCHAR(50) NOT NULL,
MODIFY COLUMN email VARCHAR(100) NOT NULL;

-- 错误的插入语句
INSERT INTO users (username, email) VALUES (NULL, 'user@example.com');

-- 正确的插入语句
INSERT INTO users (username, email) VALUES ('user123', 'user@example.com');

-- 错误的更新语句
UPDATE users SET email = NULL WHERE id = 1;

-- 正确的更新语句
UPDATE users SET email = 'newemail@example.com' WHERE id = 1;