CREATE TABLE parent (id INT NOT NULL,PRIMARY KEY (id)
) ENGINE=INNODB;


CREATE TABLE child (id INT,parent_id INT,INDEX par_ind (parent_id),FOREIGN KEY (parent_id) REFERENCES parent(id)
                      ON DELETE CASCADE
) ENGINE=INNODB;

我不明白将ENGINE = INNODB放在这里的意义,为什么我们使用ON DELETE CASCADE？