存在actor表，包含如下列信息：
CREATE TABLE IF NOT EXISTS actor (
actor_id smallint(5) NOT NULL PRIMARY KEY,
first_name varchar(45) NOT NULL,
last_name varchar(45) NOT NULL,
last_update timestamp NOT NULL DEFAULT (datetime(‘now‘,‘localtime‘)));
现在在last_update后面新增加一列名字为create_date, 类型为datetime, NOT NULL，默认值为‘0000-00-00 00:00:00’
SQL:
alter table actor add column create_date datetime not null default ‘0000-00-00 00:00:00‘
ALTER TABLE .. ADD COULUMN
