mysql设置create_time update_time自动获取时间

DROP TABLE IF EXISTS comments;
CREATE TABLE comments (
id int(11) NOT NULL AUTO_INCREMENT,
postid int(11) DEFAULT NULL,
content varchar(1024) DEFAULT NULL,
fromid int(11) DEFAULT NULL,
touserid int(11) DEFAULT NULL,
parentid int(11) DEFAULT NULL,
create_time timestamp NOT NULL DEFAULT CURRENT_TIMESTAMP,
update_time timestamp NULL DEFAULT NULL ON UPDATE CURRENT_TIMESTAMP,
PRIMARY KEY (id)
) ENGINE=InnoDB AUTO_INCREMENT=5 DEFAULT CHARSET=utf8mb4;