MySQL日常练习——多表查询

1、创建DDL——学生表——成绩表

CREATE TABLE `student` (
  `id` int(11) NOT NULL AUTO_INCREMENT COMMENT '学号',
  `createDate` datetime DEFAULT NULL,
  `userName` varchar(20) DEFAULT NULL,
  `pwd` varchar(36) DEFAULT NULL,
  `phone` varchar(11) DEFAULT NULL,
  `age` tinyint(3) unsigned DEFAULT NULL,
  `sex` char(2) DEFAULT '男',
  `introduce` varchar(255) DEFAULT NULL,
  PRIMARY KEY (`id`)
) ENGINE=InnoDB AUTO_INCREMENT=1 DEFAULT CHARSET=utf8;
 
CREATE TABLE `result` (
  `rid` int(11) NOT NULL AUTO_INCREMENT COMMENT '成绩编号',
  `testName` varchar(255) DEFAULT NULL COMMENT '测试名称',
  `score` double(4,2) DEFAULT NULL COMMENT '成绩',
  `studentId` int(11) DEFAULT NULL COMMENT '学生id',
  PRIMARY KEY (`rid`),
  KEY `studentId` (`studentId`),
  CONSTRAINT `result_ibfk_1` FOREIGN KEY (`studentId`) REFERENCES `student` (`id`) ON DELETE NO ACTION ON UPDATE NO ACTION
) ENGINE=InnoDB AUTO_INCREMENT=1 DEFAULT CHARSET=utf8;

2、创建DML——输入相关数据 

insert into student values(0,'2024-02-25 10:10:10','赵灵儿','123',
'15612345678',16,'女','逍遥哥哥，你终于找到我了。');
insert into student values(0,'2024-02-25 10:10:10','王语嫣','123',
'15612345678',17,'女','慕容复，我和你不共戴天。');
insert into student values(0,'2024-02-25 10:10:10','龙姑娘','123',
'15612345678',22,'女','我想过过过儿过过的日子。');
insert into student values(0,'2024-02-25 10:10:10','杨过','123',
'15612345678',18,'男','一遇杨过误终身。');
insert into student values(0,'2024-02-25 10:10:10','杨逍','123',
'15612345678',27,'男','杨过跟程英的大儿子。');
insert into student (userName,age,introduce)values('黄衣女子',26,'杨过与龙姑娘的大女儿。');
 
insert into result values(0,'政治',96,1);
insert into result values(0,'地理',86,5);
insert into result values(0,'政治',77,3);
insert into result values(0,'英语',99,3);
insert into result values(0,'历史',22,6);
insert into result values(0,'化学',69,2);
insert into result values(0,'Java',79,5);
insert into result values(0,'Java',67,6);
insert into result values(0,'政治',97,2);
insert into result values(0,'物理',76,6);
insert into result values(0,'物理',82,2);
insert into result values(0,'化学',56,4);

3、DQL——进行多表查询

SELECT userName,testName,score 
from result,student 
where result.studentId=student.id;
#笛卡尔积M*N

 效果如果所示。

select userName,
count(score) '参与考试的数量',
avg(score) '每人平均分数' ,
sum(score) '每人的总分' 
from result,student
where result.studentId=student.id
GROUP BY userName;

 效果如图所示。其中运用了GROUP BY 语句

内容仅供参考，具体还要亲自实践。希望大家能越来越好，谢谢！！