mysql中的on delete cascade

表类型必须是innodb，外键需要索引。

例如：
create table comment(
id INT PRIMARY KEY AUTO_INCREMENT,
userid VARCHAR(20) NOT NULL,
username VARCHAR(20) NOT NULL,
content VARCHAR(200) NOT NULL,
addtime datetime NOT NULL,
photoid INT NOT NULL,
INDEX idx_photo (photoid),
FOREIGN KEY (photoid) REFERENCES photo(id) ON DELETE CASCADE)TYPE=INNODB;