mysql如何表达字段不为空

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

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

INSERT INTO users (username, email) VALUES ('john_doe', 'john@example.com');

INSERT INTO users (username, email) VALUES (NULL, 'john@example.com'); -- 这将导致错误

SET @username = 'john_doe';
SET @email = 'john@example.com';

IF @username IS NOT NULL AND @email IS NOT NULL THEN
    INSERT INTO users (username, email) VALUES (@username, @email);
ELSE
    SELECT 'Error: Username and Email cannot be null';
END IF;