mysql设置默认值不起作用

CREATE TABLE example (
    id INT AUTO_INCREMENT PRIMARY KEY,
    name VARCHAR(255),
    created_at TIMESTAMP DEFAULT CURRENT_TIMESTAMP
);

INSERT INTO example (name) VALUES ('John Doe');

ALTER TABLE example ALTER COLUMN created_at SET DEFAULT CURRENT_TIMESTAMP;

CREATE TABLE example (
    id INT AUTO_INCREMENT PRIMARY KEY,
    name VARCHAR(255),
    age INT DEFAULT 18
);

CREATE TABLE example (
    id INT AUTO_INCREMENT PRIMARY KEY,
    name VARCHAR(255),
    created_at TIMESTAMP DEFAULT CURRENT_TIMESTAMP
) ENGINE=InnoDB;

DELIMITER $$
CREATE TRIGGER before_example_insert
BEFORE INSERT ON example
FOR EACH ROW
BEGIN
    SET NEW.created_at = NOW();
END$$
DELIMITER ;