mysql 3枚のテーブルleft join

1.     
   
DROP TABLE IF EXISTS `student`;
CREATE TABLE `student` (
  `id` int(11) NOT NULL AUTO_INCREMENT,
  `name` varchar(30) DEFAULT NULL,
  `classid` varchar(30) DEFAULT NULL,
  PRIMARY KEY (`id`)
) ENGINE=InnoDB AUTO_INCREMENT=6 DEFAULT CHARSET=utf8;

-- ----------------------------
-- Records of student
-- ----------------------------
INSERT INTO `student` VALUES ('1', '  ', '1');
INSERT INTO `student` VALUES ('2', '  ', '1');
INSERT INTO `student` VALUES ('3', '  ', '2');
INSERT INTO `student` VALUES ('4', '  ', '2');
INSERT INTO `student` VALUES ('5', '  ', '2');

   
DROP TABLE IF EXISTS `class1`;
CREATE TABLE `class1` (
  `id` int(11) NOT NULL AUTO_INCREMENT,
  `name` varchar(30) DEFAULT NULL,
  PRIMARY KEY (`id`)
) ENGINE=InnoDB AUTO_INCREMENT=4 DEFAULT CHARSET=utf8;

-- ----------------------------
-- Records of class1
-- ----------------------------
INSERT INTO `class1` VALUES ('1', '  ');
INSERT INTO `class1` VALUES ('2', '  ');
INSERT INTO `class1` VALUES ('3', '  ');

   
DROP TABLE IF EXISTS `cj`;
CREATE TABLE `cj` (
  `id` int(11) NOT NULL AUTO_INCREMENT,
  `stid` int(11) DEFAULT NULL,
  `score` int(255) DEFAULT NULL,
  PRIMARY KEY (`id`)
) ENGINE=InnoDB AUTO_INCREMENT=6 DEFAULT CHARSET=utf8;

-- ----------------------------
-- Records of cj
-- ----------------------------
INSERT INTO `cj` VALUES ('1', '1', '80');
INSERT INTO `cj` VALUES ('2', '2', '10');
INSERT INTO `cj` VALUES ('3', '3', '60');
INSERT INTO `cj` VALUES ('4', '4', '70');
INSERT INTO `cj` VALUES ('5', '5', '65');

**2.    left join    (          )
select c.`name` as className,s.`name` as stuName,j.score as stuScore 
from(class1 c LEFT JOIN student s on c.id=s.classid) 
LEFT JOIN cj j 
on s.id=j.stid
GROUP BY c.`name`;
**