mysql 一对多关联查询 练习

场景：假设一个学生有多门课程，一门课程有多个学生（虽然两者是多对多关系，但本次假设为一对多）。

学生表：

SET FOREIGN_KEY_CHECKS=0;

-- ----------------------------
-- Table structure for `student`
-- ----------------------------
DROP TABLE IF EXISTS `student`;
CREATE TABLE `student` (
  `ID` int(22) NOT NULL,
  `StuName` varchar(22) DEFAULT NULL,
  `StuAge` int(22) DEFAULT NULL,
  `Cid` int(22) DEFAULT NULL,
  PRIMARY KEY (`ID`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8;

-- ----------------------------
-- Records of student
-- ----------------------------
INSERT INTO `student` VALUES ('1', '张三', '66', '3');
INSERT INTO `student` VALUES ('2', '李四', '77', '3');
INSERT INTO `student` VALUES ('4', '王五', '25', '4');

课程表：

SET FOREIGN_KEY_CHECKS=0;

-- ----------------------------
-- Table structure for `course`
-- ----------------------------
DROP TABLE IF EXISTS `course`;
CREATE TABLE `course` (
  `ID` int(22) NOT NULL,
  `CouName` varchar(22) DEFAULT NULL,
  PRIMARY KEY (`ID`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8;

-- ----------------------------
-- Records of course
-- ----------------------------
INSERT INTO `course` VALUES ('1', '语文');
INSERT INTO `course` VALUES ('2', '数学');
INSERT INTO `course` VALUES ('3', '英语');
INSERT INTO `course` VALUES ('4', '地理');
INSERT INTO `course` VALUES ('5', '政治');
INSERT INTO `course` VALUES ('6', '历史');
INSERT INTO `course` VALUES ('7', '化学');
INSERT INTO `course` VALUES ('8', '物理');

 

需求 1：查询学生名对应的课程名

select s.StuName,c.CouName from Student s INNER JOIN Course c ON s.Cid = c.ID;

需求2：查询每门课程的选修人数

SELECT c.CouName,COUNT(DISTINCT s.StuName) AS '选修人数' FROM Student s RIGHT JOIN Course c ON s.Cid = c.ID
GROUP BY c.CouName;