1.建表和数据

学生表

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', '小王', '1');

INSERT INTO `student` VALUES ('2', '小明', '1');

INSERT INTO `student` VALUES ('3', '小李', '2');

INSERT INTO `student` VALUES ('4', '小刘', '2');

INSERT INTO `student` VALUES ('5', '小多', '2');

班级表

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', '一班');

INSERT INTO `class1` VALUES ('2', '二班');

INSERT INTO `class1` VALUES ('3', '三班');

成绩表

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');

**2.三张表格left join关联查询(以班级为依据条件查询)

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`;

**