设有如下语句: CREATE TABLE tb_booking( num INT NOT NULL AUTO_INCREMENT PRIMARY KEY, cno CHAR(10) NOT NULL, rno CHAR(10) NOT NULL, scheduleS DATETIME NULL, scheduleE DATETIME NULL, currentdate DATETIME NULL, FOREIGN KEY(cno) REFERENCES tb_customer(cno), FOREIGN KEY (rno) REFERENCES tb_room(rno)); 关于上述语句,以下叙述中错误的是